ELECTRICAL  EQUIPMENT 


McGraw-Hill  BookContpany 


Electrical  World         The  Engineering  and  Mining  Journal 
Engineering  Record  Engineering  News 

Railway  A^e  G azottx?  American  Machinist 

Signal  knginoer  American  Engineer 

Electric  Railway  Journal  Coal  Age 

Metallurgical  and  Chemical  Engineering  Power 


ELECTRICAL  EQUIPMENT 

ITS  SELECTION  AND  ARRANGEMENT 

WITH  SPECIAL  REFERENCE  TO  FACTORIES, 
SHOPS  AND  INDUSTRIAL  PLANTS 


BY 
HAROLD  W.  BROWN,  B.  S.,  M.  M.  E. 

DEPARTMENT  OF  ELECTRICAL   ENGINEERING,   CORNELL   UNIVERSITY 


FIRST  EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

239  WEST  39TH  STREET.    NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  LTD. 
6  &  8  BOUVERIE  ST.,  E.  C. 

1917 


COPYRIGHT,  1917,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


*'  •-;  *,- 


.  ;  ..,.•  'OVZ*  J   ;  *' 


THE  MAPLE  PRESS  YORK  PA 


INTRODUCTION 

A  working  knowledge  of  electrical  engineering  is  becoming  of 
greater  importance  every  day  as  the  applications  of  electrical 
apparatus  increase.  The  usual  course  of  instruction  given  to 
mechanical  engineers  includes  a  discussion  of  the  theory  of  opera- 
tion of  the  various  electrical  machines  but  goes  no  further.  It 
is  often  said  in  criticism  of  college  instruction  that  the  student 
learns  more  of  engineering  in  the  first  three  months  of  practice 
than  he  does  during  his  whole  college  career.  What  he  really 
does  when  he  begins  his  practice  is  to  work  on  a  limited  number  of 
problems,  to  the  solution  of  which  he  has  to  bring  the  sum  total 
of  his  experience;  and  thus  he  gradually  gains  confidence  in 
himself. 

The  mechanical  engineers  at  Cornell  University  cover  the  usual 
course  on  the  principles  of  electrical  engineering  during  their 
junior  year,  and  it  was  considered  desirable  that  the  work  of 
the  senior  year  include  one  or  two  pretentious  projects,  consist- 
ing of  the  selection  and  arrangement  of  all  the  electrical  equip- 
ment for  an  industrial  plant,  such  as  a  machine  shop  or  a  cement 
plant — problems  that  require  for  their  solution  a  comprehensive 
rather  than  a  detailed  knowledge  of  electrical  apparatus.  It 
was  soon  found,  however,  that  the  student  lacked  the  broad  point 
of  view  that  comes  from  practice.  He  knew  for  example  that 
direct-current  motors  were  good  for  speed  adjustment  and  that 
alternating-current  motors  were  essentially  constant-speed  ma- 
chines, but  he  was  not  able  to  reach  the  conclusion  that  there- 
fore direct-current  supply  was  desirable  for  machine  shops  with 
many  variable-speed  tools. 

The  set  of  notes  prepared  by  Mr.  Brown  to  guide  the  student 
in  his  work  contained  so  much  useful  information  not  to  be  found 
in  books  that  it  was  considered  desirable  to  offer  them  to  the 
engineering  profession.  There  are  innumerable  books  for  wire- 
men  and  for  the  shop  mechanic,  but  there  was  no  book  on  the 
market  written  specially  to  guide  the  mechanical  engineer  in  the 
selection  of  his  electrical  equipment. 

While  it  was  considered  not  only  advisable  but  necessary  to 
give  extensive  references  to  the  literature  of  the  subject,  it 


349467 


vi  INTRODUCTION 

was  recognized  that  the  mechanical  engineer  would  not  have  an 
extensive  library  of  electrical  texts,  nor  would  he  be  familiar 
with  the  electrical  periodicals.  It  was  therefore  decided  that, 
except  in  rare  cases,  the  references  would  not  go  beyond  one  of 
the  standard  texts  in  addition  to  the  electrical  handbooks. 
If  the  student  can  be  trained  to  use  the  matter  contained  in  the 
handbooks  in  an  intelligent  manner,  and  also  to  check  up  his 
theory  from  a  reliable  text,  he  will  approach  the  dreaded  electrical 
problems  with  great  confidence.  Our  experience  with  this  course 
at  Cornell  has  given  us  great  satisfaction. 

ALEXANDER  GRAY, 

Head  of  Electrical  Engineering  Department, 
Cornell  University. 


PREFACE 

A  great  mass  of  electrical  data  is  now  available  in  the  various 
handbooks,  and  other  technical  literature.  The  engineer  must 
not  only  have  such  data  at  hand — he  must  know  how  to  use  them. 
The  purpose  of  this  book  is  to  show  how  to  apply  the  available 
data,  and  the  principles  laid  down  in  textbooks,  to  the  equipping 
of  shops,  factories  and  industrial  plants.  Numerical  examples 
are  worked  out  illustrating  these  applications;  and  in  addition 
a  progressive  series  of  problems  is  placed  at  the  end  of  the  book. 
Both  the  text  and  the  problems  are  drawn  largely  from  the  au- 
thor's experience  as  engineer  with  the  Westinghouse  Electric  & 
Manufacturing  Co.,  in  the  Detail  and  Switchboard  Divisions. 

I  acknowledge  with  thanks  the  privilege  extended  by  the 
McGraw-Hill  Book  Co.,  Inc.,  publishers,  to  make  use  of  the 
material  in  the  Standard  Handbook,1  and  a  similar  privilege 
extended  by  John  Wiley  &  Sons,  Inc.,  with  reference  to  the 
American  Handbook.1  Only  condensed  data  and  brief  state- 
ments of  theory  are  included  in  the  text;  references  are  given 
throughout  the  book  to  fuller  data  in  these  two  handbooks,  and 
to  fuller  description  and  theory  as  given  in  Gray's  "Principles 
and  Practice  of  Electrical  Engineering".1 

It  gives  me  pleasure  to  express  my  thanks  to  Professor  Alex- 
ander Gray,  Head  of  the  Department  of  Electrical  Engineering, 
for  suggesting  the  publication  of  this  material,  for  painstaking 
reading  of  manuscript  and  proof,  and  for  important  suggestions, 
as  to  both  details  and  general  form  of  presentation.  I  am  glad 
to  acknowledge  also  my  obligation  for  valuable  suggestions  and 
data,  to  members  of  the  Engineering  and  Sales  Departments  of 
the  Westinghouse  Electric  &  Manufacturing  Co. ;  of  the  Engineer- 
ing Department  of  the  National  Lamp  Works  of  the  General 
Electric  Co. ;  of  the  Sales  Department  of  the  American  Steel  and 
Wire  Co.;  also  to  Mr.  W.  H.  Kniskern,  General  Manager  of  the 
Cayuga  Cement  Corporation;  and  to  Mr.  R.  A.  Hunt,  Power  and 
Electrical  Engineer  at  the  Sayre  Shops  of  the  Lehigh  Valley 
Railroad. 

H.  W.  B. 

CORNELL  UNIVERSITY, 
January,  1916. 

^ee  footnote,  page  1. 

vii 


CONTENTS 

PAQB 

INTRODUCTION v 

PREFACE vii 

CHAPTER  I 

THE  CIRCUITS  OP  POWER  PLANTS  AND  DISTRIBUTION  SYSTEMS  ...  1 

Diagrams  of  Electrical  Connections. 3 

Rules  for  Representing  Wiring 4 

Conventions  for  Representing  Apparatus    .    !    .    .    .    .    .    .  \    .    .  6 

Notes  and  Labels .   .    .    .  .'.    t 6 

CHAPTER  II 

THE  REQUISITES  OP  POWER  PLANTS  AND  DISTRIBUTING  SYSTEMS  ...  7 

Safety  to  Operators  and  Equipment 8 

Continuity  of  Service 9 

First  Cost,  Fixed  Charges,  and  Operating  Cost 10 

Voltage  Variation 12 

Adaptability  to  All  Required  Loads 13 

General  Appearance  and  Environment 14 

CHAPTER  III 

CHOICE  OP  SYSTEM 15 

A.C.  versus  D.C.  Systems 15 

Number  of  Phases 16 

Frequency 17 

Voltage.    .....    ,...'  .    .    .    .    .'  ........    ;•',-  .    .    .    .  18 

CHAPTER  IV 

D.C.  MOTORS ,    .    .    ;    .    .  22 

Voltage .'....  22 

Locations  Requiring  an  Enclosed  Motor 22 

Motor  Rating  and  Allowable  Overload 23 

Speed  Regulation ? 24 

Speed  Adjustment 25 

CHAPTER  V 

A.C.  MOTORS 28 

Types  Available '.    .  •./'.    ....    .-.,..  ;.    .*....  28 

Voltage,  Frequency  and  Phases 28 

Location ,  ..x  .;.,'••«•  ^9 

Operation  at  Various  Loads .    .    .    .  29 

Starting  Torque 31 

ix 


CONTENTS 

PAGE 

Regulation V  ..  ^  --.".'..'.  32 

Speed  Adjustment «".'-." 33 

Motor  Applications <    .    .  \.    .   .    .   .    .  33 

CHAPTER  VI 

MOTOR-GENERATORS,  CONVERTERS  AND  RECTIFIERS 34 

Converting  A.C.  to  D.C 35 

Raising  or  Lowering  D.C.  Voltage 38 

CHAPTER  VII 

TRANSFORMERS  AND  AUTO-TRANSFORMERS 43 

Applications  and  Operation  of  Transformers 43 

Auto-transformers 46 

Grouping  of  Transformers  and  Auto-transformers 48 

CHAPTER  VIII 

STORAGE  BATTERIES. , 51 

Comparison  of  Types  of  Storage  Batteries 51 

Cost 51 

Space  Occupied  and  Weight 52 

Durability  and  Repairs 52 

Current  Discharging  Rate 52 

Current  Charging  Rate 53 

Voltage 53 

Efficiency 54 

Applications  to  Stationary  Service 55 

In  the  Generating  Station 55 

In  the  Battery  Sub-station 56 

On  Circuits  that  are  Entirely  Distinct 56 

Applications  to  Portable  Service 57 

Automobile  Lighting,  Ignition  and  Starting 58 

Electric  Automobiles,  Battery  Trucks  and  Battery  Locomotives   .  58 

Train  Lighting 59 

CHAPTER  IX 

ILLUMINATION 61 

The  Essentials .-.,......  61 

Illumination  Intensity .   . 61 

Glare V  .' 67 

Color 67 

Shadows 67 

Three  Kinds  of  Illumination 68 

Computations 68 

CHAPTER  X 

D.C.  TRANSMISSION  AND  DISTRIBUTION  SYSTEMS 71 

Voltage  Drop 71 

Motor  Circuits.  71 


CONTENTS  xi 

PAGE 

Lighting  Circuits 71 

Two-wire  System 72 

Ground  or  Rail  Return V  .'  ,  ..   ;   .   .   i'V..  73 

Multiple  Voltage  Systems  ......   ^ 74 

Economical  Size  of  Wire 74 

Variable  Current V  . 78 

Safe  Size  of  Wire 79 

Conclusions 79 

Table  of  Data  on  Electrical  Conductors 80 

Notes  on  the  Table.    ...;'' 82 

CHAPTER  XI 

A. C.  TRANSMISSION  AND  DISTRIBUTION.   .    .  v.' •'*-.;   .   .   »  £  .   ;   ,    .  85 

Voltage  Drop   . ;-.  :.   .   .    ....  85 

Reactance  in  a  Single-phase  Circuit 85 

Power  Factor  on  Single  Phase  . 86 

Polyphase  Circuits 87 

Economy  and  Safety .  90 

Conclusions 90 

CHAPTER  XII 

D.C.  GENERATORS 92 

Characteristics 92 

Regulation  Curve.  .    .  :.    *    .    . 92 

Efficiency 93 

Load  Rating 94 

Parallel  Operation 94 

Cost  and  Available  Sizes 96 

Number  and  Size  of  Generators 96 

Kilowatt  Capacity  of  Plant 96 

Allowance  for  Accidents  and  Repairs 97 

Number  of  Generators 97 

CHAPTER  XIII 

A.C.  GENERATORS f~r  » 100 

Various  Classifications 100 

Phases  and  Phase  Connections 100 

Frequency 101 

Speed  and  Prime  Mover 102 

Voltage 102 

Revolving  Field  and  Revolving  Armature 102 

Characteristics 103 

Regulation 103 

Efficiency 103 

Load  Rating 104 

Requisites  for  Plant  Operation ^   .   .    .  104 

Regulation  of  Prime  Movers  and  Alternators. 104 


xii  CONTENTS 

PAGE 

Synchronizing ,  ;   ,• 105 

Connections,  Switches  and  Meters ,  .    .- 106 

Excitation  and  Voltage  Regulation  .    .    .    .»  .    ..  W  .......  107 

Cost ,    ,    .    .   •'    .    .  108 

CHAPTER  XIV 

REGULATING  TRANSFORMERS 109 

Constant  Current  Regulating  Transformer 109 

Induction  Voltage  Regulator Ill 

CHAPTER  XV 

INSTRUMENT  TRANSFORMERS 114 

Voltage  Transformers 114 

Current  Transformers 116 

Advantages  of  Using  Instrument  Transformers 120 

CHAPTER  XVI 

CONTROLLING  AND  REGULATING  EQUIPMENT 121 

Circuit  Opening  and  Closing  Equipment 121 

Knife  Switches 121 

Oil  Switches 123 

Disconnecting  Switches 123 

Control  Switches 123 

Rheostats  Controlling  Motors  and  Generators 124 

Automatic  Regulating  Equipment 126 

Generator  Voltage  Regulator 126 

Voltage  Regulating  Relay 127 

Line-drop  Compensator 128 

CHAPTER  XVII 

CIRCUIT-BREAKING  EQUIPMENT 130 

Fuses 130 

Circuit-breakers 131 

Rated  Ampere  Capacity 131 

Ultimate  Breaking  Capacity 131 

Oil  Circuit-breakers 134 

Carbon  Circuit-breakers , '.••. .  ,, 135 

Protective  Relays 136 

Time  Limit  ....    ...    .'..    .    %  .    .    .    .   ."•..'.'  •-    •    •  137 

Applications 139 

CHAPTER  XVIII 

LIGHTNING  ARRESTER  EQUIPMENT 142 

Multigap  Arrester,  ..    .    .   '. 142 

Horn-gap  Arrester 143 

Magnetic  Blowout  Arrester 144 

Condenser  Arrester  .  .144 


CONTENTS  xiii 

PAGE 

Multipath  Arrester *..... 144 

Aluminum  Arrester .„.,....  145 

Relative  Merits  of  Arresters ,.....;    .  145 

Ground  Connections  .    .    .    ." ,    .    .    .    r~.    .  147 

CHAPTER  XIX 

MEASURING  AND  INDICATING  APPARATUS 148 

Meters  and  the  Quantities  Measured 148 

Polyphase  Wattmeters  and  Watthour  Meters. 148 

Power  Factor  Meters  .'  ;  ,  j  . ...  ..  .  •. ..v  \ ''.;  .  .' 149 

Synchronizing  Apparatus  .  .  .  /:,.*/....  .  .  , 150 

Frequency  Meters 151 

Ground  Detecting  Apparatus 151 

Characteristics  of  Meters ..."..,...  .  .  .  152 

The  Scale V.  .  ...  .  .  .  .  .  ...  .  152 

Causes  of  Errors  .  .  .  .  . '  .  .  154 

Meter  Switching  Devices .  .  ...  ....  .  159 

Voltmeter  Plugs  and  Receptacles 159 

Synchronizing  Plugs  and  Receptacles 160 

Ammeter  Switches 160 

Ground  Detector  Switches . 161 

Meter  Applications .  ..-...'..' 162 

D.C.  Switchboards .  ,  .  .  .  .  .*.  .: 162 

Three-phase  Switchboards ....'. 162 

CHAPTER  XX 

MOTOR  APPLICATIONS.    .    .   ,    . 166 

Table  of  Kinds  of  Motors  .    . 167 

Table  of  Sizes  of  Motors.   .    ..!'.    .    .    . .    .    .169 

Notes  on  the  Table.    .    .."  .    .    .    .........    .    .    .    .    .    .   178 

CHAPTER  XXI 

COSTS ..   ; .   .   .  >    ...  185 

Generators  and  Motors * 185 

Switchboard  Meters V  ...    .   186 

Instrument  Transformers  and  Compensators .    .    .    .191 

Relays 193 

Switches  and  Circuit-breakers 195 

Transformers .^-  r  r 198 

Lightning  Arresters .    .    ...    .    . .    .    .    .    ...    .   199 

Current  and  Voltage  Regulators  .    .    .    .    .    .    .    .    .    .    ...    .    .    .  200 

Plug  and  Instrument  Switches .    .    .    .    .    .-.    .    .    .  200 

CHAPTER  XXII 

PROBLEMS .    .v  .    .  202 

INDEX.  .  221 


ELECTRICAL  EQUIPMENT 

ITS  SELECTION  AND  ARRANGEMENT 

CHAPTER  I 

THE  CIRCUITS  OF  POWER  PLANTS  AND 
DISTRIBUTION  SYSTEMS1 2 

This  chapter  is  intended  to  give  a  general  survey  of  the  kinds 
of  electric  circuits  in  common  use,  and  the  customary  ways  of 
representing  the  circuits  by  diagrams.  In  plants  of  any  con- 
siderable size,  practically  every  circuit  leads  to  or  from  a  set  of 
buses,  as  illustrated  in  Fig.  1.  The  generators  furnish  power  to 
the  buses,  and  feeders  take  it  to  its  destination.  Thus  it  is 
possible  for  the  generators  to  operate  in  parallel,  and  to  share  in 
the  fluctuation  of  the  load  on  any  feeder.  The  circuits  most 
commonly  found  in  practice,  feeding  to  and  from  these  various 
buses,  are: 

D.C.  generator  circuits. 

D.C.  power  and  lighting  feeders. 

A.C.  generator  circuits. 

A.C.  power  feeders. 

A.C.  constant-potential  lighting  feeders. 

Series  lighting  circuits. 

Exciter  circuits. 

1  Throughout  the  text  the  following  abbreviations  are  used  in  references: 
G.  =  Principles  and  Practice  of  Electrical  Engineering,  by  Alexander 

Gray.  The  numbers  following  G.  refer  to  paragraphs.  (First  Edit  ion,  1914, 
McGraw-Hill  Book  Co.,  Inc.) 

S.  =  Standard  Handbook  for  Electrical  Engineers.  The  numbers  pre- 
ceding colons  refer  to  sections;  those  following  colons  refer  to  paragraphs. 
In  addition  to  the  references  cited,  a  bibliography  is  given  at  the  end  of 
nearly  every  section,  and  at  the  ends  of  'many  of  the  individual  articles. 
(Fourth  Edition,  1915.  McGraw-Hill  Book  Co.,  Inc.) 

A.  =  American  Handbook  for  Electrical  Engineers.  The  numbers  refer 
to  pages.  A  bibliography  is  given  at  the  end  of  nearly  every  article,  and 
cross  references  to  related  subjects  are  given  at  the  beginning.  (First  Edi- 
tion, 1914,  John  Wiley  &  Sons.,  Inc.) 

2  G.  193-197,  Parallel  operation  of  D.C.  generators. 

S.  10  : 765-768,  799-802,  D.C.  and  A.C.  switching  connections. 
A.  pp.  1474-1478,  1494,  Switchboards  and  switching  connections. 

1 


2  ELECTRICAL  EQUIPMENT 

Each  of  these  circuits  should  be  controlled  by  a  switch,  and  pro- 
tected if  necessary  by  a  fuse  or  circuit-breaker.  In  addition,  it 
is  necessary,  for  the  intelligent  control  of  the  plant,  to  make 


D.C.  Buses  (usually  110  or  220  Volts) 


Carbon  Circuit 
Breakers 

(  Sometimes  Replaced 
by  Fuses) 


nife  Switches 


D.C.  Power  Feeders 


D.C.  Generator  Circuits 

FIG.  1. — D.C.  generator  circuits,  feeders  and  buses. 

Showing  how  D.C.  generators  deliver  current  to  the  buses,  and  the  buses  distribute  it 
to  the  several  feeders. 


3  Phase  Power  Buses 


|  Exciter  Field  Eheo. 
Exciters 


FIG.  2. — A.C.  Generator  circuits,  feeders  and  buses. 

If  the  exciters  are  to  operate  in  parallel,  an  equalizer  is  to  be  added  as  in  Fig.  1. 


measurements  of  current,  voltage,  power,  energy,  power  factor 
and  frequency,  or  some  of  them,  on  some  if  not  all  of  these 
circuits.  The  economic  considerations  determining  the  kind  of 


THE  CIRCUITS  OF  POWER  PLANTS  3 

system  to  be  installed  are  outlined  in  the  next  two  chapters. 
In  later  chapters  the  equipment  is  taken  up  in  detail. 

D.C.  generator  and  feeder  circuits  are  illustrated  in  Fig.  1, 
and  A.C.  circuits  in  Fig.  2.  In  large  power  plants,  the  connec- 
tions become  much  more  complicated,  but  still  the  entire  arrange- 
ment is  based  on  that  shown  in  these  simple  diagrams.  The 
connections  as  indicated  should  be  studied  in  detail.  A  complete 
diagram  includes  not  only  the  power  circuits  shown  here,  but 


Each  Bus  1  Strip  3"x  X 


FIG.  3. — Ammeters,  ammeter  switches  and  current  transformers  on  three 

circuits. 

Illustrating  relative  widths  of  lines,  spacing  between  lines,  and  between  groups,  and 
method  of  designating  size  of  wire,  if  necessary.  The  significance  ofithe  various  connec- 
tions will  be  better  understood  after  a  study  of  Chapter  XIX. 


also  the  connections  to  meters,  meter  switching  devices,  and 
overload  trip-coils  of  circuit-breakers.  In  many  cases  this  equip- 
ment is  not  connected  directly  to  the  line,  but  to  the  secondaries 
of  instrument  transformers,  whose  primaries  are  connected  to 
the  line.  In  Fig.  3  are  several  current  transformers  connected 
to  ammeters.  The  switching  arrangement  shown  is  discussed 
further  in  Chapter  XIX. 


DIAGRAMS  OF  ELECTRICAL  CONNECTIONS 

A  good  diagram  of  connections  is  important,  from  the  begin- 
ning to  the  end  of  the  development  of  a  power  system.  Conven- 
tional forms  and  methods  are  adopted,  which  represent  the 
apparatus  and  connections  in  one  way  or  another.  In  some  cases 
the  apparatus  should  be  represented  more  as  it  appears  to  the 
eye,  and  in  others  it  is  better  to  emphasize  theoretical  relation- 
ships. We  consider  three  features  of  diagrams;  namely:  (a) 


4  ELECTRICAL  EQUIPMENT 

rules  for  representing  wiring;  (6)  conventions  for  representing 
apparatus;  and  (c)  notes  and  labels. 

(a)  Rules  for  Representing  Wiring. — The  following  rules  for 
drawing  lines  that  represent  wiring  will  be  found  conducive  to 
clearness  and  compactness  of  the  diagram,  and  ease  of  drawing: 

1.  Lines  are  to  be  drawn  with  no  more  turns  (angles)  than  are 
necessary;  they  are  to  be  drawn  vertically  and  horizontally,  ex- 
cept that  short  lines  may  be  made  slanting,  where  it  is  particularly 
advantageous,  as  in  one  of  the  diagrams  of  the  three-phase  gen- 
erator, Fig.  4c. 

2.  Lines  must  not  cross  other  lines  more  than  is  necessary. 

3.  The  width  of  each  line  is  to  indicate  what  is  the  general  use 
of  the  conductor  (see  Table  I).     It  is  not  always  practicable  to 
represent  all  the  various  sizes  of  wire  by  widths  of  lines,  because 
only  a  few  widths  of  lines  can  be  distinguished  clearly  in  the 
ordinary  diagram.     If  the  sizes  of  wire  are  to  be  given  on  the 
diagram,  they  may   be  put  in  a  table,  or  marked  alongside 
the  line,  as  in  Fig.  3. 


TABLE  I. — WIDTHS  AND  SPACING  OF  LINES. 

The  dimensions  given  are  suitable  for  drawings.  They  may  be  varied 
somewhat,  depending  on  the  nature  of  the  diagram.  For  diagrams 
printed  from  plates,  each  dimension  may  be  divided  by  2. 


Lines  representing 

Width  of  ink 
line  (inches) 

Spacing,    center   to  center, 
between    lines 

Of  the  same 
group  (inches) 

Of  consecutive 
groups  (inches) 

Small  wiring  for  meters,  instrument 
transformers,      relays,      trip-coils, 
D.C.   generator  and  motor  fields 
(usually  about  No.  10  B.  &  S.  wire). 
Outlines  of  all  apparatus  (not  used 
for  wires)  

0.005 
0.01 

0.02 

0.035 
0.05 

He 

H 

•He 
M 

y* 

M 

% 
y* 

Leads  for  exciter  armatures,   A.C. 
generator   and  motor   fields;   also 
buses  for  auxiliary  circuits  

Leads  for  power  circuits  (i.e.,  gen- 
erator and  motor  armatures,  trans- 
formers, feeders,  etc.);  also  exciter 
buses                ...           .    .      .    . 

Ordinary  power  buses  

THE  CIRCUITS  OF  POWER  PLANTS 


5 


4.  The  space  from  the  middle  of  one  line  to  the  middle  of  the 
next  should  be  from  two  to  ten  times  the  width  of  the  line.  The 
wider  the  line  and  the  more  accurate  the  drawing,  the  less  rela- 
tive spacing  is  required. 


(a)  (6) 

D.O.  Generator  or  Motor 


(c)  (d) 

3-Phase  Generator  or 

Synchronous  Motor 


Squirrel  Wound 

Cage  Rotor 

(a)  (h) 

3-Phase  Induction  Motors 


(/) 

2  Phase  Generator  or 
Synchronous  Motor 


I  1UU 

f  rfrff 

«)  (j)    (k) 

Knife  Switches 


ffl 


(o) 

(1)    (m)     (n)       on  switch 

Carbon   Circuit- Breakers    or  Circuit 
Breaker 


FIG.  4.— Conventional    representations    of    several    kinds    of    electrical 

equipment. 

(a)  and  (6)  show  two  ways  of  representing  D.C.  machines,  (a)  shows  the  theoretical 
relationships  and  (6)  is  nearer  the  actual  appearance,  (c)  and  (e)  correspond  to  (a),  and 
(d)  and  (/)  to  (6),  representing  A.C.  machines,  (g)  and  (h)  differ  in  that  the  squirrel-cage 
motor  (g)  has  no  rheostat  connected  to  the  rotor,  such  as  the  machine  with  a  wound  rotor 
has,  as  shown  in  (h). 


5.  Not  more  than  three  or  four  lines  should  be  drawn  in  a 
group.     (Usually  a  group  consists  of  one  circuit.     See  Fig.  3.) 
Spacing  between  groups  should  be  at  least  twice  the  spacing 
between  lines  in  a  group. 

6.  A  dot  is  used  to  indicate  where  one  conductor  is  connected 
electrically  to  another,  as  in  Fig.  3.     If  the  dot  is  so  used, 


6  ELECTRICAL  EQUIPMENT 

semicircular  curves  or  " jumpers"  should  not  be  used  to  indicate 
that  wires  are  not  connected. 

(6)  Conventions  for  Representing  Apparatus. — Several  useful 
conventions  are  given  in  Fig.  4.  Such  conventional  forms  must 
sometimes  be  modified  on  account  of  changes  in  apparatus  or  in 
the  purpose  of  the  diagram. 

(c)  Notes  and  labels  should  be  added  wherever  they  make  the 
diagram  easier  to  understand,  or  easier  to  draw. 

1.  Conventional  forms  should  be  labelled  wherever  there  can 
be  any  doubt  as  to  their  meaning.     Sometimes  they  are  labelled 
by  abbreviations,  and  a  list  of  abbreviations  is  appended  to  the 
diagram. 

2.  It  is  well  to  label  circuits — e.g.,  "Lighting  Feeder,"  " Power 
Feeder  to  Woodshop,"  "Feeder  to  XYZ  Substation." 

3.  Notes  can  be  added  in  many  cases,  to  save  drawing  several 
duplications  of  circuits,  as: 

"Total  of  5  Generator  Circuits  Like  This." 

"2  Such  Feeders  to  Woodshop  and  1  to  Paint  Shop." 

4.  Reference  notes  should  be  given,  indicating  where  detail 
diagrams,  and  diagrams  of  related  installations  can  be  found,  as : 

"See  p.  47  for  Details  of  Motor-starter  Connections." 
"See  Dwg.   2,732  for  Diagram  of  Connections  of  Machine 
Shop." 


CHAPTER  II 

THE  REQUISITES  OF  POWER  PLANTS  AND 
DISTRIBUTION  SYSTEMS1 

Whether  a  power  plant  is  part  of  an  industrial  establishment 
or  is  a  commercial  plant  furnishing  power  to  customers,  the 
requisites  for  a  good  stable  investment  are  as  indicated  in  the 
following  outline,  which  is  put  in  convenient  form  for  ready  ref- 
erence. The  reference  letters  and  Roman  numerals  refer  to  the 
fuller  discussion,  which  begins  on  the  next  page. 

I.  Safety  to  operators  and  equipment,  which  requires: 

(a)  Voltage  not  unnecessarily  high. 

(6)  Adequate  insulation  of  lines  and  equipment. 

(c)  Protection  against  lightning  and  other  excessive  voltages. 

(d)  Automatic  protection  against  grounds,   short-circuits, 
and  overloads. 

II.  Continuity  of  service,  which  requires: 

(a)  All  that  is  required  for  safety. 

(6)  Duplication  of  all  essential  equipment. 

(c)  Circuit-breakers    that    do  not  operate  instantly,   and 

some  interlocking  arrangement  that  keeps  one  breaker 

in  when  another  goes  out. 

III.  Small  first  cost,  fixed  charges  and  operating  cost,  which 
require: 

(a)  All  that  is  required  for  safety. 

(6)  Voltage  neither  too  high  nor  too  low. 

(c)  Labor-saving  apparatus  without  unnecessary  complica- 
tions. 

(d)  Installation  of  no  unnecessary  equipment. 

(e)  Generating  and  other  units  neither  too  large  nor  too 
small. 

1  S.  Sections  10  to  13;  Power  Plants,  Distribution  and  Wiring. 
A.  pp.  1087,  1089,  1119,  1462,  1463;  Power  Stations  and  Substations. 
A.  pp.  251,  352,  363,  1657,  1891,  Distribution  and  Wiring. 

7 


8  ELECTRICAL  EQUIPMENT 

(/)  Units  of  high  efficiency,  operating  at  about  their  maxi- 
mum efficiency. 

(g)  Equipment  having  little  depreciation. 

(k)  Equipment  requiring  little  outlay  for  upkeep  and 
repairs. 

({)  Conditions  of  low  interest,  insurance,  and  taxes. 

IV.  Small  per  cent,  voltage  variation,  which  requires: 

(a),  (6)  High  line  voltage  or  large  line  wires  for  small  per 
cent  drop. 

(c),  (d)  Suitable  compounding  or  voltage  regulation  of  D.C. 
generators,  voltage  regulation  of  A.C.  generators,  or  reg- 
ulation of  feeder  voltage,  or  any  combination  of  these 
methods  of  regulation. 

(e)  Power  factor  of  the  A.C.  load  as  high  as  possible. 

V.  Adaptability  to  all  required  loads,  which  requires: 

(a)  Sufficient  total  capacity  of  generating  units. 
(6)  Capacities  of  some  or  all  the  units  not  much  in  excess  of 
the  minimum  load. 

(c)  Allowance  for  expansion. 

(d)  Ammeters  and  wattmeters  whose  full-scale  indications 
are  sufficient  for  all  ordinary  overloads,  and  whose  in- 
dications at   customary  loads  are  readable  with   fair 
accuracy. 

VI.  Good  appearance,  which  encourages  keeping  the  plant  in 
good  condition,  and  requires: 

(a)  Consideration  of  appearance  in  selecting  equipment. 
(6)  Orderly  layout  of  switchboard  and  machines. 

(c)  Good  building. 

(d)  Well-arranged  natural  and  artificial  lighting. 

I.  SAFETY  TO  OPERATORS  AND  EQUIPMENT 

(a)  High  voltages  are  undesirable  if  the  employees  working 
around  the  circuits  are  not  familiar  with  electricity.  Usually 
there  is  no  advantage  that  would  warrant  a  voltage  much  above 
550,  in  a  plant  employing  non-electrical  men  (see  Chapter  III, 
p.  19,  for  customary  voltages). 

(6)  Insulation. — All  parts  of  the  system  should  be  insulated 
well  enough  so  that  there  is  no  danger  of  breakdown  to  ground, 
nor  from  one  wire  to  another.  For  all  inside  wiring,  the  insula- 


THE  REQUISITES  OF  POWER  PLANTS  9 

tion  should  be  in  accordance  with  the  rules  of  the  National 
Electrical  Code,1  together  with  city  ordinances,  power  company 
requirements  and  local  insurance  rulings,  if  there  are  such. 

(c)  Lightning  Arresters. — When  lightning  strikes  a  line,  the 
danger  is  on  account  of  the  excessively  high  voltage  that  may 
occur  between  the  line  and  ground.     If  this  voltage  due  to  the 
lightning  is  high  enough,  the  insulation  breaks  down  at  the  weak- 
est point,  and  the  lightning  discharges  to  ground.     A  lightning 
arrester  provides  a  direct  and  easy  path  to  ground,  thereby  reduc- 
ing the  strain  on  the  insulation.     This  easy  path  is  made  opera- 
tive the  instant  the  lightning  strikes,  but  is  made  inoperative  as 
soon  as  the  strain  due  to  the  lightning  is  past  (see  Chapter  XVIII) . 

Lightning  arresters  serve  as  a  protection  against  other  high 
voltages  on  the  line,  as  well  as  against  lightning.  This  is  a  mat- 
ter of  considerable  importance  in  case  of  some  high-tension  lines 
in  which  the  voltage  surges  that  are  liable  to  occur  in  the  opera- 
tion of  the  system  become  excessive. 

(d)  Automatic  Protection    against    Grounds,   Short-circuits, 
and  Overloads. — If  a  system  is  already  grounded  at  one  point, 
the  result  of  another  ground,  on  another  phase  or  polarity,  is 
equivalent  to  a  short-circuit.     Such  a  ground  or  a  short-circuit 
is  very  much  like  a  heavy  overload,  and  is  to  be  treated  accord- 
ingly.    A  circuit-breaker  can  be  set  to  operate  when  the  current 
in  the  line  exceeds  the  maximum  safe  value,  thereby  protecting 
against  all  these  troubles  (see  Chapter  XVII). 

II.  CONTINUITY  OF  SERVICE 

(a)  Safety  Requirements. — Whatever  is  required  for  safety 
is  also  important  for  continuity  of  service,  because  a  breakdown 
is  likely  to  interrupt  the  service. 

(6)  Duplicate  Equipment. — As  far  as  practicable,  the  plant 
should  be  laid  out  so  that  if  any  one  part  is  disabled  it  need  not 
interrupt  the  entire  plant.  By  providing  at  least  one  more  of 
every  piece  of  equipment  than  is  required  for  normal  operation, 
such  danger  of  interruption  is  largely  avoided.  The  emergency 
equipment  should  be  available  on  an  instant's  notice,  for  use 
wherever  required,  if  it  is  not  actually  in  service.  Consider  two 

1  Rules  and  Requirements  of  the  National  Board  of  Fire  Underwriters 
for  Electric  Wiring  and  Apparatus;  revised  every  2  years.  These  rules  can 
be  obtained  from  any  local  insurance  inspection  office,  or  from  the  head- 
quarters in  Philadelphia. 


10  ELECTRICAL  EQUIPMENT 

cases:  (1)  A  spare  generator  takes  its  turn  at  lying  idle,  but  it 
should  be  driven  by  an  engine  or  other  prime  mover  that  can  be 
started  without  delay  (see  Chapter  XII,  p.  97).  (2)  In  case 
of  transmission  lines  and  distribution  systems,  two  lines  are  kept 
in  continuous  service,  in  parallel.  Either  line  may  then  drop 
out  of  service  on  account  of  some  fault;  the  other  line  then  carries 
the  entire  load  (see  Chapter  XVII,  p.  140) .  For  economic  reasons 
this  duplication  of  equipment  cannot  be  carried  to  an  extreme, 
and  those  responsible  for  laying  out  the  plant  must  decide  to 
what  extent  they  are  willing  to  sacrifice  continuity  of  service  in 
case  of  emergency,  in  order  to  reduce  the  first  cost  of  the  plant. 

(c)  Restricted  Operation  of  Circuit-breakers. — Unnecessary 
opening  of  the  circuit-breaker  is  to  be  avoided  whenever  pos- 
sible, so  that  the  circuit-breaker  should  be  set  for  as  large  a  cur- 
rent as  is  safe;  and  usually  it  is  better  for  the  sake  of  continuity 
of  service,  to  allow  some  time  to  elapse,  so  that  if  possible  the 
trouble  will  clear  itself  without  opening  the  breaker.  This  time 
element  is  introduced  in  many  cases  by  means  of  a  small  relay 
which  determines  the  time  when  the  circuit-breaker  is  to  open 
(see  Chapter  XVII,  p.  137). 

In  many  cases  it  is  necessary  to  lay  out  the  wiring  so  that  a 
single  short-circuit  tends  to  open  two  circuit-breakers.  It  is 
desirable  that  the  breaker  nearer  the  seat  of  trouble  should  open, 
but  that  the  other  should  remain  closed,  because  opening  it 
usually  affects  some  other  machines.  In  such  a  case  the  circuit- 
breakers  are  made  to  operate  "selectively;"  that  is,  there  is  a 
mechanical  or  electrical  interlock  that  prevents  the  one  trouble 
from  tripping  more  than  one  breaker. 

HI.  FIRST  COST,  FIXED  CHARGES,  AND  OPERATING  COST 

The  first  cost  is  the  cost  of  equipment,  including  transportation 
and  installation.  The  building,  steam  plant,  electric  plant,  and 
wiring  naturally  come  under  this  head.  Fixed  charges  are  for 
annual  depreciation,  interest  on  the  investment,  insurance,  and 
taxes.  Operating  cost  includes  labor,  fuel,  supplies,  upkeep  and 
repairs. 

(a)  Safety. — Whatever  makes  for  safety  reduces  one  element 
of  the  operating  cost,  by  saving  in  repairs. 

(b)  Voltage.— The   voltage   best   adapted  to  any  particular 
circuit  depends  on  the  extent  of  the  system.     A  high-voltage 
installation  may  cost  more  for  protection  and  insulation  than 


THE  REQUISITES  OF  POWER  PLANTS  11 

one  of  low  voltage,  but  on  long  lines  the  saving  in  copper  by 
increasing  the  voltage  is  so  great  as  to  make  the  higher  voltage 
more  economical  (see  Chapter  III). 

(c)  Labor-saving  Apparatus. — Under   this   head  is  included 
whatever  saves  time  or  special  attention,  such  as  meters,  instru- 
ment   transformers,    meter-switching    devices,    and    indicating 
lamps  (see  Chapters  XV  and  XIX). 

(d)  Unnecessary    Equipment. — The    dividing    line    between 
necessary  and  unnecessary  equipment  is  not  always  easy  to 
draw;  but  unless  every  $100  invested  brings  $10  or  $15  return 
every  year,  in  profit,  labor-saving,  or  protection,  it  is  not  usually 
a  very  good  investment.     Sometimes  meters  can  be  omitted,  and 
perhaps  even  more  often  they  can  be  replaced  by  switching  de- 
vices to  shift  an  ammeter  and  voltmeter  from  one  phase  to 
another,  and  to  shift  a  voltmeter  from  one  circuit  to  another 
(see  Chapter  XIX). 

(e)  Size  of  Generators  and  Other  Units. — Large  generators 
and  transformers  cost  less  per  kilowatt  capacity  than  small 
ones;  but  the  investment  in  spare  machines  becomes  excessive 
if  all  the  units  are  of  too  large  capacity.     Both  the  first  cost  of 
the  entire  plant  and  the  operating  cost  must  be  considered  in 
determining  the  most  economical  size  of  machines  (see  Chapters 
VII,  XII  and  XIII). 

(/)  High  efficiency  operation  is  desirable,  not  only  because  the 
cost  of  operation  is  less,  but  also  because  the  machine  runs  cooler 
at  high  efficiency. 

(g)  Depreciation. — The  total  amount  annually  chargeable  to 
depreciation  is  proportional  to  the  investment,  except  that  some 
equipment  depreciates  less  rapidly  than  other.  On  account  of 
the  smaller  depreciation,  apparatus  having  the  larger  first  cost 
is  frequently  the  more  economical. 

(h)  Upkeep  and  Repairs. — Some  apparatus  rarely  requires 
any  attention  to  keep  it  in  perfect  condition.  Other  apparatus 
is  laid  up  for  repairs  an  appreciable  part  of  the  time.  The  loss 
is  twofold:  The  cost  for  making  the  repairs,  and  the  loss  of  the 
use  of  the  apparatus  during  the  time  of  repairs. 

(i)  Interest,  Taxes,  Insurance. — Interest  and  taxes  are  essen- 
tially proportional  to  the  total  investment,  except  as  better 
rates  of  interest  are  obtainable  for  a  plant  that  is  better  pro- 
tected and  more  durable.  Insurance  depends  on  the  quality  of 
the  building  to  such  an  extent  that,  considering  the  small  cost  of 


12  ELECTRICAL  EQUIPMENT 

the  building,  the  saving  in  insurance  usually  warrants  construc- 
tion that  is  at  least  approximately  fireproof. 

IV.  VOLTAGE  VARIATION 

Except  series  lighting  circuits,  practically  all  power  and  light- 
ing apparatus  operates  from  nominally  constant-potential  sys- 
tems. Only  a  small  percentage  variation  from  constant  poten- 
tial is  allowable  in  motor  circuits,  and  on  incandescent  lighting 
circuits  it  should  be  still  smaller.  Excessive  variation  of  voltage 
with  load  on  account  of  line  drop  is  avoided  by  adopting  a  suitable 
line  voltage,  and  suitable  size  and  spacing  of  conductors. 

(a)  Line  Voltage. — If  the  amount  of  power  transmitted,  and 
the  length  of  line  remain  constant,  the  higher  the  voltage  the 
less  is  the  per  cent,  voltage  drop.  The  voltage  must,  therefore, 
be  high  enough  so  that  a  line  can  be  designed  whose  voltage  drop 
is  not  excessive,  with  reasonable  sizes  of  wire  (see  Chapter  III). 

In  case  of  A.C.  circuits  it  is  good  practice  to  use  one  voltage  for 
transmission  over  considerable  distances,  and  another  voltage 
for  local  distribution. 

(6)  Size  of  Conductors. — After  the  voltage  of  transmission 
and  distribution  has  been  fixed,  the  conductors  must  be  of  such 
size  and  spacing  that  the  drop  does  not  exceed  a  prescribed 
maximum.  On  A.C.  circuits,  line  reactance  as  well  as  resistance 
produces  voltage  drop,  but  of  course  on  D.C.  circuits  only 
resistance  is  to  be  considered  (see  Chapters  X  and  XI). 

(c)  Compounding  D.C.  Generators. — The  voltage    of  small 
shunt  generators  decreases  so  much  with  load  that  if  constant 
potential  is  at  all  important  on  D.C.  circuits,  small  generators  are 
compound-wound.     If  all  the  power  is  used  so  near  the  bus  that 
there  is  no  considerable  line  drop,  the  generator  •  may  be  flat- 
compounded —  that  is,  compounded  so  that  its  voltage  at  full- 
load  is  the  same  as  at  no-load.     But  if  all  the  power  is  trans- 
mitted to  a  distant  point,  such  that  the  per  cent,  line  drop  is 
large,   the   machine   may  be   overcompounded — that  is,   com- 
pounded so  that  the  generator  voltage  at  full-load  is  higher  than 
at  no-load,  thereby  compensating  for  the  line  drop  and  main- 
taining constant  voltage  at  the  load  (see  Chapter  XII). 

(d)  Voltage  Regulation. — It  is  necessary,  in  many  cases,  to 
provide,  external  to  the  generator,  some  means  of  voltage  regu- 
lation.    This  is  of  less  importance  on  D.C.  than  on  A.C.  circuits, 
because   A.C.   machines   are   not  generally  self -regulating.     If 


THE  REQUISITES  OF  POWER  PLANTS  13 

all  the  feeders  are  short,  the  voltage  of  the  entire  system  can  be 
kept  constant  by  a  device  called  a  voltage  regulator,  applied 
to  the  generators;  but  if  some  feeders  are  very  long,  each  one 
may  require  additional  individual  regulation  (see  Chapters  XIV 
and  XVI). 

(e)  Control  of  Power  Factor. — The  greatest  voltage  drop  due 
to  line  reactance  occurs  in  case  of  a  lagging  current  at  low  power 
factor.  With  current  at  100  per  cent,  power  factor,  the  reactance 
has  no  appreciable  effect  on  the  terminal  voltage,  and  with  a 
leading  current  the  reactance  tends  to  increase  the  terminal 
voltage  (see  Chapter  XI,  p  87).  Where  it  is  feasible,  the  load 
should  be  so  arranged  that  the  power  factor  is  about  100  per 
cent.,  thereby  eliminating  reactance  drop. 

V.  ADAPTABILITY  TO  ALL  REQUIRED  LOADS 

(a)  The  total  capacity  of  the  generating  units  must  be  large 
enough  to  carry  all  ordinary  loads  with  good  efficiency,  and  with- 
out excessive  heating,  sparking,  voltage  drop,  or  other  harmful 
effect  of  overload.  The  capacity  should  be  such  that  if  any  one 
unit  is  disabled,  the  remaining  machines  can  carry  the  load 
without  seriously  crippling  the  plant  or  endangering  the  remaining 
generators  (see  Chapters  XII  and  XIII). 

(6)  Units  for  Light  Load. — It  should  be  possible  to  run  the 
plant  at  good  efficiency,  at  the  lightest  load  that  the  plant  is 
likely  to  carry  for  any  considerable  time.  If  the  plant  has  a 
large  number  of  machines,  they  should  all  be  of  the  same  size; 
in  case  of  a  small  plant,  having  only  two  or  three  generators, 
and  operating  for  long  periods  at  a  very  light  load,  it  may  be 
advantageous  to  have  one  of  the  units  of  about  one-half  the  size 
of  the  others. 

(c)  Allowance  for  Expansion. — If  the  plant  is  of  such  a  nature 
that  the  demand  for  power  is  likely  to  increase,  the  plant  should 
be  large  enough  to  anticipate  the  increase,  or  provision  should 
be  made  for  one  or  more  additional  generators  and  related 
equipment,  to  be  installed  later. 

(d)  Meter  Capacity. — Ammeters  and  wattmeters  should  be 
of  such  capacity  that  they  will  indicate  the  largest  load  that  the 
line  is  likely  to  carry  for  any  appreciable  time;  but  the  meters 
should  not  have  too  large  capacity,  because  in  that  case  the 
deflection  is  very  small  under  normal  and  light-load  conditions, 
and  it  is  not  possible  to  take  accurate  readings. 


14  ELECTRICAL  EQUIPMENT 

VI.  GENERAL  APPEARANCE  AND  ENVIRONMENT 

This  does  not  necessarily  refer  to  ornamentation,  but  the  plant 
should  give  the  appearance  of  being  adapted  to  its  purpose.  It 
should  of  course  satisfy  stockholders  and  directors  and  the 
general  public,  but  it  should  also  be  so  designed  that  it  appeals 
to  those  who  are  operating  the  plant,  as  being  worthy  of  the  best 
of  care.  An  operator  takes  pride  in  keeping  a  plant  in  perfect 
condition,  if  he  believes  it  to  be  the  best  of  its  kind  in  the  vicinity. 

(a)  Consideration  of  Appearance  in  Selecting  Equipment.— 
The  best  manufacturers  of  electrical  equipment  take  pride 
in  their  products,  and  are  not  willing  to  send  them  out  poorly 
finished.  It  requires  only  a  small  additional  expense  to  make  a 
good  piece  of  apparatus  appear  well,  and  to  some  extent  the 
appearance  serves  as  a  guarantee  of  good  workmanship  and 
materials.  For  this  reason,  and  because  apparatus  that  is  well- 
finished  is  easier  to  keep  in  order,  it  pays  to  consider  appearance 
in  selecting  it. 

(6)  Layout  of  Switchboard  and  Machines. — Switchboard 
panels  should  be  grouped,  so  that  as  far  as  possible  all  A.C. 
generator  panels  are  in  one  group,  all  A.C.  feeder  panels  in  another, 
lighting  feeders  in  another,  etc.  This  plan  is  frequently  carried 
so  far  as  to  leave  blank  panels  for  future  additions,  rather  than 
to  install  the  additions  out  of  order.  The  machines  should,  if 
possible,  be  in  essentially  the  same  order  as  the  switchboard 
panels.  It  is  easier  for  the  operator  to  act  quickly  in  an 
emergency,  and  easier  to  give  proper  care  at  all  times,  if  the  entire 
layout  is  so  rational  that  the  operator  has  it  perfectly  in  mind. 

(c)  The  Building. — The  cost  of  the  building  is  small  compared 
with  the  cost  of  equipment,  but  the  convenience,  safety  and 
durability  of  the  building  play  no  small  part  in  the  safe  and 
economical  operation  of  the  plant. 

(d)  Natural  and  Artificial  Lighting.— The  lighting  of  the  power 
plant  should  be  good :  first,  because  every  operation  is  performed 
more  safely  and  easily  with  good  light;  and  second,  because  if 
any  part  of  the  plant  is  in  disorder  good  lighting  reveals  the  fact. 
Good  lighting,  whether  natural  or  artificial,  includes  (1)  plenty 
of  light,  (2)  uniform  distribution,  and  (3)  light  so  placed  that 
there  is  no  glare  or  blinding  effect,  due  to  strong  direct  or  reflected 
light  in  the  field  of  vision  (see  Chapter  IX). 


CHAPTER  III 
CHOICE  OF  SYSTEM 

The  first  things  to  be  decided  in  arranging  for  an  electrical 
installation  are: 

(1)  Whether  it  is  to  be  an  A.C.  or  a  D.C.  system; 

(2)  If  A.C.,  whether  a  one-,  two-,  or  three-phase  system; 

(3)  Whether  the  frequency  is  to  be  25  cycles,  60  cycles,  or 
some  frequency  that  is  not  standard; 

(4)  What  the  line  voltage  or  voltages  are  to  be. 

1.  A.C.  versus  D.C.  Systems. — If  a  storage  battery  or  any  other 
electrolytic  equipment  is  to  be  connected  to  the  system,  obviously 
a  D.C.  system  must  be  employed.  Otherwise,  the  decision  must 
be  based  on  the  relative  merits  in  the  individual  case,  because 
either  A.C.  or  D.C.  can  be  used  for  both  lighting  and  power. 
For  incandescent  lighting,  either  an  A.C.  or  a  D.C.  system  is 
satisfactory  if  the  voltage  can  be  maintained  constant.  For  arc 
lighting,  the  D.C.  has  some  advantages,  but  not  enough  to  war- 
rant its  use  if  there  are  many  motors  connected  to  the  circuit 
which  operate  better  on  A.C. 

It  is  possible  to  use  either  an  A.C.  or  a  D.C.  motor  at  will 
for  every  motor  application ;  but  there  are  some  cases,  for  example, 
in  a  powder  mill,  where  D.C.  is  very  undesirable;  and  others, 
for  example,  where  there  are  cranes  and  many  variable  speed 
machine  tools,  where  A.C.  would  be  objectionable.  In  each 
plant  where  a  choice  is  possible,  the  advantages  of  the  two  sys- 
tems must  be  balanced  against  each  other.  The  chief  advantages 
in  using  A.C.  motors  for  industrial  applications  are: 

(a)  The  possibility  of  A.C.  transmission  and  distribution  at 
higher  voltages,  and  stepping  down  to  motor  voltages. 

(6)  The  possibility  of  stepping  down  from  the  voltage  of  motor 
feeders  to  that  of  lighting  feeders  without  a  lower  voltage 
generator  or  its  equivalent. 

(c)  The  possibility  of  using  induction  motors  which  are  rugged, 
and  have  no  commutator  to  wear  out  or  introduce  fire  hazards. 

15 


16  ELECTRICAL  EQUIPMENT 

The  chief  advantages  in  B.C.  motors  are: 

(a)  The  possibility  of  automatic  speed  variation  of  series  and 
compound  motors,  without  excessive  power  lost  in  rheostats. 

(6)  The  possibility  of  speed  adjustment  in  shunt  motors  by 
field  rheostat,  thereby  obtaining  at  high  efficiency  any  number  of 
speeds  that  remain  constant  at  the  required  values,  at  all  loads. 

(c)  The  absence  of  reactance  in  line  drop. 

(d)  The  possibility  of  connecting  motors  to  the  same  circuit 
as  storage  batteries  or  other  electrolytic  apparatus. 

In  Table  XIII,  p.  167  is  a  list  of  machine  tools  and  other  in- 
dustrial motor  applications,  indicating  the  motors  that  are  usually 
preferable,  and  others  that  are  sometimes  satisfactory. 

2.  Number  of  Phases. — The  only  systems  in  general  use  are 
single-phase,  two-phase  four-wire,  two-phase  three-wire,  and 


AA/VWV1 


(d) 

FIG.  5. — Comparison  of  distributing  systems. 

(a)  Single-phase.  (6)  2-phase  4-wire.  The  two  voltages  are  equal,  (c)  2-phase  3-wire. 
The  CA  voltage  is  \/2  times  BA  or  CB.  (d)  3-phase.  The  three  voltages  are  equal. 

three-phase.  These  are  illustrated  in  Fig.  5.  The  zigzag  lines 
may  be  taken  to  represent  transformers  or  any  other  source  of 
power.  Their  angular  positions  indicate  the  phase  relations  of 
the  several  circuits. 

The  principle  advantage  of  a  single-phase  circuit  is  its  simplic- 
ity. It  requires  only  two  wires  for  transmission,  and  the  con- 
nections and  equipment  are  sometimes  a  little  simpler  than  for  a 
polyphase  circuit.  The  chief  advantages  of  a  polyphase  system 
are  that  the  size  and  cost  of  motors  and  generators  are  less,  and 
that  motor  starting  is  easier.  The  advantages  of  the  polyphase 
system  far  outweigh  those  of  the  single-phase,  for  ordinary  indus- 
trial-motor applications.  The  only  extensive  single-phase  appli- 
cation is  to  railway  motors,  which  operate  from  only  one  trolley 


CHOICE  OF  SYSTEM  17 

line  and  the  ground  return.  There  is  but  little  choice  between 
single-phase  and  polyphase  systems  for  lighting  circuits.  For 
small  currents,  the  use  of  only  two  wires  is  an  advantage ;  for  large 
currents,  the  polyphase  system  offers  a  small  saving  in  copper. 

After  it  has  been  decided  to  use  a  polyphase  system,  there  is 
but  little  choice  between  two-  and  three-phase.  The  small  ad- 
vantages of  the  several  systems  are  as  follows: 

The  two-phase  four-wire  system  has  the  advantage  over  the 
three-phase,  in  requiring  only  two  transformers,  and  in  having 
two  phases  that  are  insulated  from  each  other  and  independent 
in  their  operation. 

The  two-phase  three-wire  system  has  the  advantage  over  the 
four-wire  system,  that  the  wiring  is  simplified,  and  there  is  a 
small  saving  in  copper. 

The  three-phase  system  has  the  advantage  over  the  two-phase 
three-wire,  that  the  three  voltages  between  lines  are  equal,  and 
all  conductors  are  of  the  same  size. 

Where  a  generating  station  is  to  be  installed,  the  advantages 
of  a  three-phase  system  usually  outweigh  those  of  two-phase;  but 
if  there  is  a  power  company  from  which  power  can  be  obtained  in 
an  emergency,  the  system  used  by  that  company  should  be  con- 
sidered. Transformers  are  used  to  step  the  power  company's 
voltage  down  to  that  required  by  the  industrial  plant,  and  even 
in  case  the  power  company  should  have  a  two-phase  system,  the 
transformers  could  be  Scott-connected  (see  Chapter  VII,  page  48), 
so  that  three-phase  could  still  be  used  in  the  industrial  plant. 
However,  if  two-phase  emergency  power  could  be  obtained  with- 
out stepping  down  the  voltage,  it  might  be  well  to  introduce 
two-phase  in  the  industrial  plant,  and  save  the  transformers. 

It  should  be  remembered  that  in  transforming  it  is  always 
possible  to  change  not  only  from  two-phase  to  three-phase,  but 
also  from  three-phase  to  two-phase,  and  from  two-phase  three- 
wire  to  two-phase  four-wire. 

3.  Frequency. — In  the  choice  of  the  frequency  of  an  alternat- 
ing-current system,  some  considerations  make  a  low  frequency 
advantageous,  and  others  a  high  frequency. 

Transformers,  generators,  synchronous  motors,  and  induction 
motors  can  be  made  smaller,  and  at  less  cost,  if  they  are  to  operate 
on  a  high  than  if  on  a  low  frequency.  But  very  slow-speed  induc- 
tion motors  are  more  satisfactory  on  25  than  on  60  cycles,  on 
account  of  the  higher  power  factor  that  can  be  obtained  on  25 


18  ELECTRICAL  EQUIPMENT 

cycles.  The  cost  of  a  25-cycle  transformer  or  induction  motor 
is  usually  from  25  to  50  per  cent,  higher  than  a  corresponding 
60-cycle  equipment,  and  a  25-cycle  generator  or  synchronous 
motor  sometimes  costs  20  per  cent,  more  than  a  corresponding 
60-cycle  machine. 

Alternating-current  commutating  motors,  used  on  electric  rail- 
ways, operate  at  a  higher  power  factor  on  low  frequency.  For 
this  reason  they  are  preferably  designed  for  about  25  cycles. 

Arc  and  tungsten  lamps  flicker  visibly  on  very  low-frequency 
circuits.  It  is  better  not  to  use  them  on  less  than  50  or  60  cycles, 
on  account  of  the  resulting  eye-strain. 

Transmission  and  Distribution  Wiring. — The  voltage  drop  due 
to  inductive  reactance  of  the  line  is  greater  at  high  than  at  low 
frequency;  it  is  entirely  negligible  at  any  ordinary  frequency  if 
the  load  current  is  at  100  per  cent,  power  factor,  but  may  be  very 
large  with  load  at  low  power  factor. 

The  desirable  frequency  to  be  adopted  on  any  system  depends 
on  what  equipment  predominates.  Alternating-current  commu- 
tating motors  are  used  on  electric  railways,  but  very  little  else- 
where; slow-speed  induction  motors  are  used  only  in  steel  rolling 
mills,  and  for  other  rather  rare  applications.  For  these  motors 
a  low  frequency  is  better;  but  for  all  others  the  frequency  should 
be  higher.  The  reactance  drop  in  the  various  machines,  as  well 
as  in  the  line,  would  be  excessive  if  the  frequency  were  too  high, 
but  it  is  found  in  practice  that  at  25  to  60  cycles  this  drop  need 
not  be  excessive,  either  in  the  machines  or  in  the  line.  In 
the  United  States,  25  and  60  cycles  are  used  in  practice  almost 
exclusively,  for  ordinary  industrial  purposes.  Of  the  two,  60 
cycles  is  far  more  common  than  25,  especially  where  slow-speed 
induction  motors  and  alternating-current  commutating  motors 
are  not  employed.  Besides  these  frequencies,  15,  30,  50,  133, 
and  a  few  others  are  found  occasionally.  It  is  of  advantage  to 
use  25  or  60  cycles  wherever  possible,  because  standard  equipment 
is  made  for  these  frequencies.  If  any  other  frequency  is  adopted, 
the  delivery  of  equipment  may  be  delayed,  and  the  cost  may  be 
higher. 

4.  The  voltage  of  a  so-called  "constant-potential"  system,  or 
of  any  circuit  in  such  a  system,  depends,  chiefly,  on  (1)  the  length 
of  the  system,  (2)  the  kind  of  apparatus  that  it  feeds,  and  (3) 
the  danger  to  life  or  apparatus.  All  of  these  should  be  considered 
with  reference  to  every  circuit. 


CHOICE  OF  SYSTEM  19 

Standard  Voltages. — The  voltages  in  common  use  at  the  pres- 
ent time  are  110,  220,  440,  550,  1,100,  2,200,  4,400,  6,600,  11,- 
000,  13,200,  16,500,  22,000,  33,000,  44,000,  66,000,  88,000,  and 
110,000.  All  of  these  are  derived  from  110  volts,  by  multiplying 
by  the  factors  2,  3  and  5,  as  many  times  as  necessary.  Various 
other  voltages,  as  high  as  165,000  are  in  less  common  use.  On 
account  of  line  drop,  the  voltage  cannot  be  standard  along  the 
entire  length  of  the  line.  Either  the  beginning  or  the  end  of 
the  line  is  usually  kept  at  or  near  the  standard  voltage. 

Voltage  of  Distribution  System. — There  is  no  simple,  universal 
rule  for  the  relation  between  voltage  and  length  of  line;  but  in 
practice,  where  transformers  are  necessary,  the  line  voltage  is 
nearly  always  between  500  and  2,000  volts  per  mile,  and  is  ordi- 
narily about  1,000  volts  per  mile.  Thus  a  line  10  miles  long 
usually  has  a  voltage  of  about  10,000.  The  choice  would  be 
between  6,600  and  11,000,  which  are  standard  voltages.  It 
would  be  decided  by  considerations  of  cost,  safety,  and  voltage 
regulation  (see  Chapter  X).  However,  if  some  other  line  in  this 
same  system  would  dictate  a  higher  voltage,  sometimes  it  is  of 
advantage  to  have  the  entire  system  at  that  higher  voltage. 

If  transformers  are  installed  at  the  end  of  the  line,  it  is 
usual  practice  to  have  a  line  voltage  of  at  least  2,200,  which 
is  very  common  as  a  transmission  and  distribution  voltage. 
Wherever  it  is  possible  to  use  2,200  volts,  it  is  better  to  adhere 
to  this  standard  than  to  adopt  the  higher  voltages  which  in- 
troduce added  risks,  especially  in  populous  districts. 

Tungsten  lamps  connected  in  multiple  are  usually  used  on 
about  110  volts.  (Lamps  are  also  made  for  220  volts,  but  in  most 
cases,  at  present  prices,  the  220-volt  lamps  of  a  given  wattage  cost 
20  per  cent,  more,  and  produce  only  90  per  cent,  as  much  light.) 
In  a  plant  of  any  considerable  size,  if  only  a  220-volt  D.C. 
circuit  is  available,  some  convenient  means  can  be  provided, 
so  as  to  have  a  110-  and  220-volt  three- wire  system.  Various 
other  arrangements  can  be  employed  at  other  voltages,  to  obtain 
a  suitable  lighting  system.  In  case  alternating  current  is  avail- 
able, transformers  may  be  employed  to  provide  the  desired 
voltage  for  the  lighting  circuit. 

Arc  lamps  connected  in  multiple  on  a  D.C.  circuit  operate 
efficiently  and  satisfactorily  on  about  110  volts.  (A  little  lower 
voltage  could  be  used  if  there  were  a  standard  of  about  70  volts, 
but  the  gain  would  not  be  very  large.)  When  higher  D.C.  volt- 


20  ELECTRICAL  EQUIPMENT 

ages  than  110  are  employed,  a  relatively  large  series  resistance  is 
used  to  cut  down  the  voltage  across  the  arc,  and  the  efficiency  is 
very  much  reduced.  For  A.C.  circuits,  however,  arc  lamps  are 
regularly  made  for  110,  220  and  440  volts.  For  voltages  above 
110,  a  transformer  accompanies  the  lamp,  and  a  good  electrical 
efficiency  is  obtained,  even  on  440  volts.  Special  lamps  can  be 
obtained,  also,  to  operate  at  still  higher  voltages. 

D.C.  motors  usually  operate  on  110  or  220  volts  in  industrial 
plants.  Line  drop  is  unnecessarily  large  in  a  large  plant  at  110 
volts,  but  220  volts  is  satisfactory  if  the  motors  are  within  500 
or  1000  feet  of  the  power  plant.  A  550- volt  system  in  an  in- 
dustrial plant  has  two  disadvantages:  that  it  is  approaching  a 
dangerous  condition  if  it  is  a  plant  in  which  non-electrical  men 
are  regularly  at  work;  and  that  some  provision  must  be  made 
for  lighting,  other  than  the  customary  three-wire  system. 

A.C.  motors  may  operate  at  any  desired  voltage,  without  regard 
to  the  lighting  circuit,  because  transformers  can  be  employed 
for  the  lighting  voltage.  It  is  rarely  desirable  to  operate  the 
motors  on  less  than  220  volts.  A  440-volt  system  is  more 
common  in  any  plant  requiring  power  several  hundred  feet  from 
the  generator;  and  even  a  550-volt  system  is  sometimes  used. 

A  motor-generator  set  has  no  electrical  connections  between 
the  motor  and  the  generator,  so  that  the  voltage  of  each  machine 
may  be  whatever  is  required  to  adapt  it  to  its  circuit.  The  same 
fact  holds  for  a  dynamotor  (see  Chapter  VI,  p.  42). 

A  synchronous  converter,  used  to  convert  either  from  A.C. 
to  D.C.,  or  from  D.C.  to  A.C.,  has  only  one  winding  for  the 
A.C.  and  D.C.  circuits,  and  there  is  a  very  nearly  constant 
ratio  between  the  A.C.  and  D.C.  voltages.  This  ratio  depends 
on  the  A.C.  connections  to  the  converter,  and  is  as  follows: 

_.    ,.      A.C.  voltage 
A.C.  connection  to  converter  Ratio   p  Q  voita 

Single-phase 0.707 

Two-phase 0 . 707 

Three-phase 0.612 

Six-phase,  double-delta 0.612 

Six-phase,  diametrical 0 . 707 

The  ratio  given  is  for  ordinary  connections.  Special  connec- 
tions may  be  employed  to  obtain  other  ratios  in  case  of  the  two- 
and  six-phase  connection. 


CHOICE  OF  SYSTEM  21 

A  storage  battery,  with  or  without  a  booster,  has  a  voltage  on 
discharge  that  is  the  same  as  the  line  voltage.  The  charging 
voltage  depends  on  what  method  of  charging  is  employed.  If 
the  storage  battery  is  connected  across  only  one  side  of  a  three- 
wire  system,  its  discharge  voltage  is  only  the  lamp-line  voltage 
— that  is,  one-half  the  motor  voltage. 


CHAPTER  IV 
D.C.  MOTORS1 

In  specifying  the  motor  and  its  controlling  apparatus  for  any 
given  kind  of  service  it  is  necessary  to  determine  what  kind  of 
D.C.  circuit  is  available,  and  what  is  required  of  the  motor.  We 
should  know  in  particular: 

(1)  What  D.C.  voltage  is  available,  and  whether  it  is  a  two- 
wire  or  a  three- wire  system; 

(2)  Where  the  motor  is  located; 

(3)  What  is  the  maximum  load  on  the  motor,  and  how  the 
load  varies; 

(4)  What  automatic  variation  of  speed  with  load  is  desired  or 
allowable;  and 

(5)  What  speed  adjustment  is  required  to  be  made  by  hand  at 
the  will  of  the  operator,  under  various  conditions. 

1.  The  voltage  of  the  motor  and  of  the  line  must  be  adapted 
to  each  other.     The  best  voltage  for  industrial  plants  is  generally 
220  volts;  but  if  no  motors  are  over  a  few  hundred  feet  from  the 
generator,  110  volts  may  be  used.2 

2.  Locations  Requiring  an  Enclosed  Motor. — The  motor  should 
be  enclosed,  if  it  is  in  a  place  where  dust  would  injure  the  com- 
mutator or  interfere  with  commutation,  where  shavings  might 
catch  fire  from  the  commutator,  or  where  water  might  injure 
or  short-circuit  the  machine.     The  motor  may  be  semi-enclosed 
or  entirely  enclosed  to  keep  flying  objects  from  the  moving  parts 
of  the  machine.     A  semi-enclosed  machine  may  also  be  used  in 
some  cases  where  there  is  slight  trouble  from  dust,  shavings  or 
water,  but  not  enough  to  require  that  the  machine  be  entirely 

1  G.  Chapter  XV,  Characteristics;  Chapter  XVII,  Applications;  Chap- 
ter XVIII,  Speed  Control. 

S.  8:  157-180,  200-205,  Characteristics,  weights  and  costs;  Section  15, 
Applications. 

A.  pp.  957-971,  Characteristics,  weights  and  costs;  pp.  892-896, 972-982 
(also  see  references,  p.  972),  Applications. 

2  See  Chapter  III,  p.  20. 

22 


D.C.  MOTORS  23 

enclosed.  Enclosing  a  motor  increases  its  cost  per  horse-power, 
because  it  reduces  the  horsepower,  that  can  be  delivered  con- 
tinuously without  overheating.  See  Chapter  XX,  p.  183,  as  to 
the  effect  of  enclosing  on  the  motor  rating. 

3.  Motor  Rating  and  Allowable  Overload. — The  motor  rating 
given  by  manufacturers  is  usually  based  on  the  horsepower 
that  the  machine  will  deliver  without  causing  excessive  heating 
of  any  part1  or  excessive  sparking  at  the  commutator.  Usually 
the  rating  is  based  on  continuous  duty  (8  hr.  or  more),  but  some- 
times on  1-hr,  duty,  or  even  a  shorter  time.  On  the  short-time 
basis  the  rating  is  much  higher  than  on  continuous  duty  (see 
page  183  for  increase  of  rating).  For  example,  the  ordinary  crane 
motor  does  not  operate  more  than  a  few  minutes  at  a  time,  and 
a  M-hr-  °r  &  1-hr-  rating  is  sufficient;  so  that  the  motor  size,  weight, 
and  cost  can  be  much  less  than  if  it  were  driving  a  lathe  for 
8  hr.  If  the  load  is  intermittent  or  variable,  having  maximum 
values  of  several  times  the  average,  it  is  well  to  specify  that  the 
motor  is  to  deliver  power  according  to  a  given  time-load  curve. 
The  ratio  of  starting  torque  to  full-load  running  torque  should 
also  be  specified,  especially  if  the  motor  is  expected  to  exert  a 
very  great  starting  torque. 

After  the  full-load  rating  has  been  determined,  in  terms  of 
either  horsepower,  or  torque  and  speed,  the  full-load  current 
can  be  found  directly: 

2irtf  !T/33,000  =  h.p.  =  El  X  efficiency/746 

where  T  is  the  full-load  torque  in  pound-feet  and  N  is  the  speed 
in  r.p.m. 

Table  II  gives  reasonable  figures  for  efficiency  and  rated 
speeds.  If  the  motor  is  well-constructed  mechanically,  and 
commutation  is  good,  the  speed  may  be  increased  in  each  case 
by  field  control,  to  double  the  rated  speed.  If  the  torque  is 
one-half  as  great  at  double  the  speed,  the  power  delivered  is  the 
same  as  before,  and  the  current  input  is  practically  the  same. 
The  ventilation  is  so  much  better  at  higher  speeds  that 
most  motors  running  at  double  the  normal  speed  will  stand  120 
per  cent,  of  the  rated  armature  current;  so  that  the  torque  on 
continuous  duty  at  double  the  rated  speed  may  be  as  much  as 
60  per  cent,  of  that  at  rated  speed. 

1  For  A.I.E.E.  standard  of  allowable  temperature  rise  see  footnote,  p.  94. 


24  ELECTRICAL  EQUIPMENT 

TABLE   II. — D.C.  MOTOR   DATA   FOR  CONTINUOUS  DUTY  AT  FULL-LOAD 


Rating  in  horsepower 

Efficiency,  per  cent. 

R.p.m.1 

2 

80 

1,000 

5 

83 

800 

10 

85 

600 

20 

88 

500     . 

40 

89 

400 

100 

90 

300 

300 

93 

200 

1,000 

95 

100 

1200 


100     1000 


26  50  75  100  125  150 

Percent  of  Full  Load 

FIG.  6. — Typical  efficiency  and  speed  curves  of  D.C.  motors. 

4.  Speed  Regulation. — Fig.  6  shows  the  comparative  regula- 
tion (automatic  variation  of  speed  with  load)  of  shunt,  compound 
and  series  motors.  The  large  majority  of  industrial  applications 

1  There  are  no  definite  limitations  of  rated  speeds  of  D.C.  as  there  are 
of  A.C.  motors.  The  speeds  given  in  this  table  are  in  common  use.  The 
chief  disadvantage  of  lower  speeds  is  the  increased  cost  of  the  motor.  The 
chief  disadvantages  of  higher  speeds  are  commutation  difficulties,  and  the 
extra  gear  reduction  that  is  necessary. 


D.C.  MOTORS  25 

require  that  motor  speeds  remain  approximately  constant  under 
all  conditions  of  loading.  A  lathe  is  an  example.  See  also 
the  table  of  motor  applications  in  Chapter  XX,  page  167. 
Shunt  motors  satisfy  the  requirement  of  constant  speed  so  well 
that  they  are  used  in  all  such  cases.  (For  extremely  constant 
speed  the  motor  may  be  differentially  compounded,  but  at  the 
present  time  the  differential  motor  is  almost  obsolete.) 

A  compound  motor  is  adapted  for  operating  such  machines  as 
shears,  punches  and  crushers,  which  require  a  heavy  torque 
intermittently.  The  kinetic  energy  of  the  rotating  armature  is 
utilized  when  the  machine  slows  down  at  the  instant  of  heavy 
torque.  If  a  flywheel  is  on  the  motor  shaft,  the  available 
kinetic  energy  is  still  greater.  The  compound  motor  is  also 
adapted  to  certain  hoists  and  pumps,  and  similar  applications, 
in  which  the  speed  should  be  less  at  heavy  than  at  light  loads. 

A  series  motor  has  a  still  greater  speed  variation  with  load  than 
a  compound  motor;  it  is  in  danger  of  running  away  at  very  light 
loads.  Its  speed  regulation  is  adapted  to  hoisting  and  conveying, 
where  it  cannot  run  away;  because  it  is  always  under  the  control 
of  the  operator,  even  if  it  is  not  always  loaded.  There  are  in 
use  three  methods  of  protection  against  overspeed  of  a  series 
motor: 

(a)  Gearing  or  other  positive  connection  to  a  load  that  cannot 
be  less  than  a  safe  minimum. 

(6)  An  operator  who  is  present,  controlling  the  speed  whenever 
the  motor  is  running. 

(c)  Sufficient  resistance  in  the  motor  circuit  to  keep  down  the 
back  e.m.f.,  and  therefore  the  speed  of  the  machine.  This 
makes  the  machine  inefficient  and  the  speed  less,  especially  at 
heavy  loads.  It  is  only  suited  to  very  small  motors. 

5.  Speed  adjustment  refers  to  changes  made  at  the  will  of  the 
operator,  whereas  speed  regulation  refers  to  the  automatic  change 
in  speed  due  to  change  in  torque. 

Speed  Adjustment  by  Rheostats. — The  most  common  method 
of  speed  adjustment  is  by  a  rheostat  in  either  the  field  or  the 
armature  circuit  or  one  in  each  circuit.1  The  effects  of  these 
rheostats  are  shown  very  clearly  by  the  expression  for  speed  of 
a  D.C.  motor, 

r.p.m.  =  k  (Ea  —  IaRa}/<t> 
1  See  Chapter  XVI,  for  points  to  be  considered  in  specifying  rheostats. 


26  ELECTRICAL  EQUIPMENT 

Where  Ea  is  the  applied  voltage. 

Ia  is  armature  current  in  amperes. 

Ra  is  resistance  of  armature  circuit  in  ohms. 

<j>    is  the  flux  in  lines  per  pole. 

The  armature  rheostat  is  a  part  of  the  resistance  Ra',  when  the 
resistance  in  the  rheostat  is  increased,  the  speed  decreases,  until 
when  Rala  —  Ea  the  speed  is  zero.  It  should  be  noted  that 
when  Ra  is  large,  the  regulation  is  poor;  the  variation  of  speed  with 
load  is  excessive,  because  Rala  varies  with  7a,  and  therefore  with 
the  torque.  The  RP  power  loss  is  also  large,  so  that  this  method 
of  regulation  is  very  inefficient. 

Compare  with  the  foregoing  the  effect  of  the  field  rheostat. 
An  increase  of  its  resistance  decreases  the  field  current,  and  so 
the  flux,  <£;  and  the  speed  variation  is  in  inverse  proportion  to 
the  flux.  Since  Ra  is  now  only  the  small  armature  resistance, 
its  effect  on  the  speed  is  negligible  under  all  ordinary  conditions. 
There  is  no  large  RP  power  loss,  so  that  this  is  an  efficient  method 
of  speed  control.  An  excessive  resistance  weakens  the  field 
so  much  that  commutation  is  bad,  unless  the  motor  has  a 
commutating  field.  Motors  can  be  built  having  speed  adjust- 
ment by  field  rheostat,  by  which  the  speed  can  be  increased  from 
normal  to  twice  the  normal  speed. 

The  effect  of  an  armature  rheostat  with  any  given  current 
and  resistance  is  calculated  readily  from  the  speed  formula  given 
above;  but  the  effect  of  the  field  rheostat  cannot  be  determined 
accurately  without  knowing  how  much  the  field  is  saturated. 
As  an  approximate  guide,  it  may  be  assumed  that  for  every  2 
per  cent,  increase  in  field  circuit  resistance  the  speed  is  increased 
1  per  cent. 

Speed  control  by  armature  rheostat  can  be  applied  to  shunt, 
compound  and  series  motors.  A  field  rheostat  can  be  used  on 
the  shunt  field  of  either  a  shunt  or  a  compound  motor.  A  similar 
field  control  of  a  series  motor  is  accomplished  by  connecting  a 
rheostat  in  parallel  with  the  series  field.  Increasing  the  resistance 
of  such  a  rheostat  increases  the  field  strength,  by  sending  more 
of  the  current  through  the  field  winding. 

Multiple-voltage  Speed  Control. — The  difficulties  of  speed  con- 
trol by  armature  rheostat  are  overcome  by  the  multiple- 
voltage  system.  For  slower  than  normal  speed  the  armature  is 
connected  across  a  lower  voltage,  while  the  field  remains  un- 


D.C.  MOTORS  27 

changed.  The  motor  runs  at  essentially  the  same  reduced  speed 
at  light  load  as  at  full  load,  and  has  a  reasonably  good  efficiency. 
The  difficulty  with  this  arrangement  is  that  some  form  of  motor- 
generator  set  must  be  provided  to  obtain  the  additional  voltages, 
and  additional  wiring  is  necessary  wherever  the  multiple  voltage 
is  required.  The  only  multiple  voltage  in  general  use  at  the 
present  time  is  the  three-wire  system  having  equal  voltages  on 
the  two  sides.  For  example,  on  a  110-  and  220-volt  three- wire 
system,  if  a  motor  armature  is  connected  across  110  volts,  the 
speed  is  one-half  of  the  normal  speed  which  the  motor  has  when 
across  220  volts.  Intermediate  speeds  are  obtained  by  connect- 
ing the  armature  to  110  volts  and  weakening  the  field  by  a 
rheostat.  Still  higher  speeds,  as  high  as  double  the  normal,  are 
obtained  by  connecting  the  armature  across  220  volts  and 
weakening  the  field  by  the  rheostat. 


CHAPTER  V 
A.C.  MOTORS1 

Types  Available. — Of  all  A.C.  motors,  the  squirrel-cage  and 
phase-wound2  induction  motors  are  the  ones  used  most  exten- 
sively for  industrial  purposes  at  the  present  time.  Synchronous 
motors  and  various  forms  of  commutating  motors  are  used  to  a 
limited  extent  for  power-factor  adjustment,  but  the  simplicity 
and  ruggedness  of  the  induction  motors  make  them  desirable 
wherever  it  is  practicable  to  use  them.  The  present  discussion 
will  be  limited  to  the  two  kinds  of  induction  motors.  In  pro- 
viding A.C.  motors,  the  following  should  be  considered: 

(1)  The  voltage,  frequency  and  number  of  phases; 

(2)  Where  the  motor  is  located; 

(3)  What  the  maximum  load  is  on  the  motor,  and  how  the 
load  varies; 

(4)  What  starting  torque  is  required,  compared  with  full-load 
torque; 

(5)  Whether  it  is  necessary  that  the  speed  at  full-load  be  ap- 
preciably less  than  at  no-load,  and  if  so  how  much;  and 

(6)  Whether  any  speed  adjustment  is  required  to  be  made  by 
the  operator,  and  if  so  how  much. 

1.  Voltage,  Frequency  and  Phases. — The  motor  should  be 
designed  for  the  same  number  of  phases,  the  same  voltage  and  the 
same  frequency  as  the  supply. 

If  applied  voltage     >motor  voltage      1  in)n  logg  Jg  ^s^ 

If  applied  frequency  <  motor  frequency  J 

If  applied  voltage     <  motor  voltage      1  maximum  torque  is 

If  applied  frequency  >  motor  frequency  j      too  low. 

Single-phase  induction  motors  are  of  value  where  it  is  impor- 
tant to  use  only  two  wires  for  distribution  of  power;  but  poly- 
phase motors  are  to  be  preferred,  particularly  on  account  of 

1  G.  Chapter  XXXVI,  Theory  and  Characteristics;  Chapter  XXXVII, 
Applications;  Chapter  XXXVIII,  Single-phase  motors. 

S.  7  : 205,  215-221,  271-285,  Characteristics;    Section  15,  Applications. 
A.  pp.  983-988,  1005,  1007-1013,  Characteristics,  Costs  and  Weights; 
pp.  892-896,  972-982,  also  references  on  p.  972,  Applications. 

2  Otherwise  known  as  "wound-rotor"  or  "slip-ring"  motors. 

28 


A.C.  MOTORS 


29 


special  arrangements  that  are  necessary  for  a  good  starting  torque 
of  the  single-phase  motors. 

There  is  no  essential  difference  in  operation  between  two- 
phase  and  three-phase  motors.  If  it  is  necessary  to  operate  a 
two-phase  motor  on  three-phase,  the  Scott  connection  of  trans- 
formers, or  preferably  auto-transformers,  should  be  employed  (see 
Fig.  23  h  and  i,  p.  49  for  the  Scott  connection,  and  Chapter 
VII,  p.  46  for  the  advantage  of  using  auto-transformers  instead 
of  transformers). 

2.  The  location  has  less  effect  on  the  kind  of  induction  motor 
than  of  a  D.C.  motor,  because  the  induction  motor  has  no  com- 
mutator; and  therefore  no  commutator  trouble  due  to  dust,  and 
no  fire  hazard  due  to  ignition  from  commutator  sparking.  The 
frame  of  the  motor  surrounds  the  winding  to  such  an  extent  that 
injury  from  water  or  flying  objects  is  almost  impossible.  If  the 
motor  is  subjected  to  continued  dampness  or  acid  fumes,  the 
coils  should  be  treated  with  a  special  varnish.  A  phase- wound 
motor  with  external  starting  resistance  requires  slip-rings,  which 
should  be  enclosed  if  the  machine  is  in  a  very  dusty  location. 
The  bearings  of  any  induction  motor  in  a  dusty  place  should 
also  be  well  protected  from  dust. 


100 


7. — Charactistic  curves  of  a  10-hp.,  3-phase,  220-volt,  60-cycle  induc- 
tion motor. 


3.  Operation  at  Various  Loads. — Increasing  the  load  applied 
to  an  induction  motor  affects  the  motor  operation,  as  to  speed, 
power  factor,  efficiency  and  current.  Referring  to  Fig.  7,  it 


30 


ELECTRICAL  EQUIPMENT 


will  be  seen  that  for  the  highest  efficiency  and  power  factor,  the 
load  should  be  neither  very  large  nor  very  small.  The  speed 
decreases  gradually  with  increase  of  load  until  a  certain  maximum 
horsepower  output  is  reached.  The  motor  would  stop  if  a  torque 
were  applied  much  beyond  the  value  for  maximum  horsepower. 
Usually  the  motor  is  made  large  enough  so  that  the  maximum 
possible  torque  is  100  per  cent,  more  than  the  motor  will  be 
required  to  deliver. 

The  full-load  current  of  a  three-phase   induction  motor  is 
obtained  directly  from  the  expression, 

27rA/T/33,000  =  h.p.  =  ^%EI  cos  6  X  efficiency/746  (1) 
where  N  is  the  speed  in  r.p.m.,  T  is  the  torque  in  pound-feet, 
cos  6  is  the  power  factor,  E  is  the  voltage  between  lines  and  / 
the  amperes  per  motor  lead.  Equation  (1)  holds  for  single-phase 
and  two-phase  motors,  if  the  constant,  -\/3,  is  replaced  by  1 
and  2,  respectively.  Table  III  gives  reasonable  values  for  full- 
load  efficiency  and  power  factor,  no-load  and  full-load  speeds 
and  per  cent,  slip,  for  typical  60-cycle  and  25-cycle  motors. 


TABLE  III. — TYPICAL  DATA  AT  FULL-LOAD  AND  NO-LOAD,  ON  SQUIRREL- 
CAGE  AND  PHASE-WOUND  INDUCTION  MOTORS1 


•  i 

1 

£-* 

60-cycle  motor 

25-cycle  motor 

Horse- 

^-d 

m 

^1 

power 
Rating 

|f 

sl3 
§  §2 
\r 

11 

No-load  or  syn- 
chronous r.p.m. 

Full-load 
r.p.m. 

No-load  or 
synchronous 
r.p.m. 

Full-load 
r.p.m. 

.PV3 

PH 

* 

1 

82 

78 

5.5 

1,800 

1,700 

(1,800-600) 

5 

85 

86 

5.5 

1,800 

1,700 

(1,800-600) 

10 

87 

88 

5.0 

1,200 

1,140 

750 

712 

(1,800-600) 

25 

89 

89 

4.0 

1,200 
(1,800-600) 

1,150 

750 

720 

50 

89 

89 

3.5 

900 

870 

750 

725 

(1,800-600) 

100 

90 

90 

3.5 

600 

580 

500 

485 

(1,800-514) 

500 

91 

91 

3.0 

600 

582 

375 

365 

1,000 

92 

92 

2.5 

450 

440 

250 

244 

1  The  speeds  given  in  parentheses  are  the  highest  and  lowest  for  which 
the  sizes  given  are  ordinarily  made. 


A.C.  MOTORS 


31 


Both  the  usual  load  and  the  maximum  must  be  considered, 
with  reference  to  all  the  operating  characteristics,  in  specifying 
a  motor.  Also  the  heating  on  short-time  overloads  must 
not  be  excessive  (see  Chapter  XX,  p.  183,  for  allowable  short- 
time  overloads). 

4.  Starting  Torque.  —  Induction  motors  are  made  with  two 
kinds  of  rotating  elements,  or  rotors.     One  of  these,  the  wound 
rotor,  has  a  winding  in  the  rotor 
slots,  with  leads  brought  out  to 
slip-rings.       A    rheostat    con- 
nected to  the  slip-rings  as  in 
Fig.  8,  is  mounted  outside  the 
motor.     It   is   used   to   insert 


Ante. 

transformer 


Primary  or  Stator  Winding 

Connected  Directly  Through 

a  Switch  to  the  Line 


Slip  Rings 
to  which  the 
Secondary  or 
Eotor  Connects 

FIG.  8. — Induction  motor  with 
wound  rotor  connected  to  a  rheo- 
stat. 


FIG.  9. — Squirrel-cage  induction 
motor  with  auto-transformers  and 
switches  for  starting  on  low  voltage. 

The  diagram  shows  the  theoretical  con- 
nections. The  circuit-breaker  is  opened 
by  hand  before  the  motor  is  started. 
Usually  the  switches  and  circuit-breaker 
are  combined  so  that  the  change  from 
starting  to  running  position  is  made  by  a 
single  switching  operation. 


resistance  in  the  rotor  winding  when  the  motor  is  being 
started.  The  effect  of  the  resistance  is  to  reduce  the  cur- 
rent, and  to  increase  the  power  factor.  Such  an  increase  of  power 
factor  is  important,  because  otherwise  it  is  very  low  at  starting, 
and  the  necessary  starting  current  is  very  high.  Where  a  rheostat 
in  the  motor  secondary  controls  the  starting  current,  the  rela- 
tion between  torque  and  current  is  the  same  as  at  full  speed 
(see  Fig.  7). 

The  other  type  of  rotor  is  called  a  squirrel-cage.  It  has  heavy 
conductors,  short-circuited  on  themselves.  If  the  motor  were 
started  at  full  voltage,  the  current  in  the  rotor  (and  also  in  the 
stator)  would  be  excessive.  The  starting  voltage  is  stepped 
down  by  auto- transformers  as  in  Fig.  9  (see  Chapter  VII,  page 


32 


ELECTRICAL  EQUIPMENT 


46;  also  G.  307,  332).  The  smaller  the  starting  voltage,  the 
smaller  is  the  starting  current.  If  the  voltage  is  too  low,  the 
motor  will  not  start.  Table  IV  gives  the  starting  voltages  and 
the  resulting  currents  that  produce  certain  starting  torques. 
The  voltage  should  be  sufficient  for  starting  under  the  worst 
conditions;  but  if  it  is  kept  as  small  as  practicable  it  prevents 
excessively  large  starting  currents.  For  example,  if  in  starting, 
a  motor  requires  1.1  times  its  full-load  torque,  then  we  find  the 
starting  voltage  must  be  80  per  cent,  of  the  full  voltage,  and  the 
primary  of  the  auto-transformer  must  take  from  the  line  3.85 
times  the  motor  full-load  current.  In  a  motor  with  a  wound  rotor 
the  current  above  full-load  is  nearly  proportional  to  the  torque, 
as  shown  by  Fig.  7,  so  that  it  would  have  started  with  about 
1.1  times  full-load  current. 

TABLE  IV. — CURRENT  AND  VOLTAGE  REQUIRED  FOR  STARTING  TYPICAL 

SQUIRREL-CAGE  INDUCTION  MOTORS  AT  VARIOUS  STARTING  TORQUES 

(These  figures  are  subject  to  some  variations,  depending  on  the  purpose  for 

which  the  motor  is  designed) 


Per  cent,  of  full- 
load  torque 

Per  cent,  of  full  line 
voltage 

Per  cent,  of  full-load 
current  in  motor 

Per   cent,   of  full-load 
current  in   primary 
leads  to  auto-trans- 
former 

27 

40 

240 

100 

60 

60 

360 

220 

110 

80 

480 

385 

170 

100 

600 

600 

5.  Speed  Regulation. — Ordinary  induction  motors,  whether 
squirrel-cage  or  phase-wound,  correspond  approximately  to 
shunt  motors  in  their  speed  regulation.  This  is  in  accordance 
with  the  usual  requirements  for  industrial  applications;  but  as 
applied  to  a  punch  press,  for  example,  there  is  an  advantage  in 
decreasing  the  speed  when  the  torque  is  very  great.  This  is 
accomplished  by  permanently  inserting  resistance  in  the  second- 
ary of  either  a  squirrel-cage  or  a  phase-wound  motor,  as  men- 
tioned in  a  preceding  paragraph.  This  produces  a  speed 
characteristic  similar  to  that  of  a  D.C.  compound  motor,  but  it 
is  obtained  at  the  expense  of  efficiency.  The  more  resistance  in 
the  secondary,  the  greater  the  speed  reduction  and  the  less  the 
efficiency. 


A.C.  MOTORS 


33 


6.  Speed  adjustment  requires  either  a  special  motor  whose 
synchronous  speed  can  be  changed  (A.  p.  977,  Multi-speed  Induc- 
tion Motors),  or  a  phase- wound  motor  with  a  rheostat  to  be  in- 
serted in  the  secondary,  just  as  is  done  in  motor  starting.  The 
first  of  these  is  not  used  so  extensively  as  the  second,  although 
by  changing  the  synchronous  speed  a  high  efficiency  is  main- 
tained, and  the  speed  is  very  nearly  constant.  Where  the  speed 
adjustment  is  by  the  phase- wound  motor,  the  rheostat  carries 
the  secondary  current  continuously — or  for  as  long  a  time  as  the 
reduced  speed  is  required.  Such  a  rheostat  must,  therefore, 
have  a  heavier  conductor  than  one  used  only  for  starting.  If 
the  rheostat  is  used  for  both  starting  and  speed  adjustment,  the 
first  few  steps,  which  are  necessary  for  a  speed  adjustment,  may 
be  heavy  enough  for  continuous  use;  but  more  resistance  is  re- 
quired for  starting,  and  the  additional  steps  need  only  be  heavy 
enough  for  short-time  use. 

Motor  Applications. — Some  of  the  customary  applications  of 
induction  motors  are  listed  in  Chapter  XX,  page  167.  Table 
V  reviews  the  special  advantage  of  each  motor. 


TABLE  V. — THE  KINDS  OF  SERVICE  FOR  WHICH  THE  SEVERAL  A.C.  MOTORS 
ARE  PARTICULARLY  ADAPTED 


Kind   of   motor 


Specially  adapted  for 


Example  of  application 


Phase-wound. . . 

Squirrel-cage... 
Synchronous  — 


Single-phase 
induction . . 


25-cycle . 
60-cycle. 


Large  starting  torque. 
Hand  control  of  speed. 

Applications  where   phase- 
wound  is  not  necessary. 

Control  of  power  factor. 
Small  starting  torque. 
Very  constant  speed. 


Where     only     single-phase 
circuit  is  available. 

Very  slow  speed. 

Applications      where       25 
cycles  is  not  necessary. 


Tube  mill  in  cement  plant. 
Elevator. 

Wood-working     machinery 
that  starts  without  load. 

Motor-generator  set. 
Motor-generator  set. 

Frequency-changing  motor- 
generator  set. 

Small  fans. 


Steel  rolling  mill. 
Nearly  everywhere. 


CHAPTER  VI 

MOTOR-GENERATORS,  CONVERTERS  AND 
RECTIFIERS 

Electricity  appearing  in  one  form  is  converted  to  another  in  a 
variety  of  ways: 

1.  A. C.  is  converted  from  one  voltage  to  another  by  an  ordinary 
transformer  (see  Chapter  VII). 

2.  A.C.  is  converted  from  constant  potential  to  constant  current 
by  a  constant-current  regulating  transformer  (see  Chapter  XIV) . 

3.  A.C.  is  converted  from  one  number  of  phases  to  another  by 
a  suitable  combination  of  transformers,  such  as  the  Scott  connec- 
tion (see  Chapter  VII) . 

4.  A.C.  is  converted  from  one  frequency  to  another  by  a  motor- 
generator  set  called  a  frequency  changer,  in  which  the  generator 
has  not  the  same  number  of  poles  as  the  synchronous  or  induction 
motor,  so  that  the  frequency  of  the  generator  is  different  from 
that  of  the  motor  circuit;  for  example,  if  a  25-cycle  supply  is 
available  and  a  60-cycle  system  is  desired,  a  25-cycle  motor  is 
used  to  drive  a  60-cycle  generator. 

5.  D.C.is  changed  to  A.C.  (a)  by  a  shunt  motor  driving  an  A.C. • 
generator,  or  (b)  by  an  inverted  synchronous  converter. 

6.  A.C.  is  changed  to  D.C.  (a)  by  a  synchronous  or  induction 
motor  driving  a  shunt  or  compound  generator,  (6)  by  a  synchron- 
ous converter,  (c)  by  a  mercury  rectifier,  or  (d)  by  a  vibrating 
rectifier. 

7.  D.C.  is  changed  from  one  voltage  to   another   (a)  by  two 
machines  comprising  a  balancer-set,  connected  in  series  across  the 
higher  voltage,  with  a  neutral  or  low -voltage  lead  brought  out 
from  the  connection  between  machines;   (6)   by  a  three- wire 
generator  which  is  provided  with  one  or  two  coils  by  which  the 
neutral  voltage  of  a  three-wire  system  is  established;  (c)  by  a 
motor-generator  set  in  which  the  motor  and  generator  are  de- 
signed for  different  voltages;  or  (d)  by  a  dynamotor  consisting 
of  a  machine  having  two  distinct  circuits — one  for  the  higher  and 
one  for  the  lower  voltage,  each  circuit  having  its  own  complete 
winding  and  commutator. 

34 


CONVERTERS 


35 


This  list  includes  only  the  conversions  that  are  most  frequently 
seen  in  industrial  plants,  and  only  the  means  most  frequently 
employed  for  making  them.  The  first  three  of  the  conversions 
listed  are  accomplished  by  transformers,  and  are  discussed  in 
Chapter  VII.  The  fourth,  frequency  changing,1  and  the  fifth, 
converting  D.C.  into  A.C.,2  are  not  discussed  further,  because 
they  are  comparatively  rare.  "We  shall  consider  the  sixth  and 
seventh  in  this  chapter. 

6.  Converting  A.C.  to  D.C.  is  most  often  desirable  where  a 
relatively  long  line  requires  A.C.  transmission,  and  a  storage  bat- 
tery or  a  number  of  variable-speed  motors  call  for  D.C.  applica- 
tions. Each  of  the  means  of  conversion  has  its  advantage,  and 
is  the  best  to  use  in  certain  cases. 

(a)  A  motor-generator  set  may  consist  of  either  a  self-starting 
synchronous  or  an  induction  motor,  and  either  a  shunt  or  a 


From  A.C.  Source 

and 
Starting  Apparatus 


From  A.C.  Source 

and 
Starting  Apparatus 


From  D.C.  Source 
and  Field  Switch 


FIG.  10. — Motor-generator  sets. 

(a)  Synchronous  motor-generator  set,  requiring  connection  to  D.C.  for  field  excitation. 
It  may  be  started  by  an  additional  induction  motor  winding.  (6)  Induction  motor-gener- 
ator set,  having  a  squirrel-cage  motor,  which  requires  no  connections  to  the  rotor. 

compound  generator.3  The  induction  motor  is  more  rugged 
and  more  easily  started,  and  does  not  require  synchronizing  nor 
D.C.  field  excitation.  The  synchronous  motor  can  be  operated 
at  unity  power  factor  or  with  a  leading  current,  and  has  no  speed 
variation  between  no-load  and  full-load.  Large  motors  of 
either  kind,  with  suitable  windings,  can  be  connected  to  the  line 
without  transformers,  if  the  line  voltage  is  not  over  13,200. 
For  small  motors  the  voltage  must  be  lower.  Self-starting 
synchronous  motors  are  usually  used  in  motor-generator  sets  of 
500  kw.  or  more;  induction  motors  are  ordinarily  used  for  100  kw. 
or  less  on  account  of  their  simplicity;  and  between  100  and  500 

1 S.  7  : 346-369;  A.  pp.  951,  952. 

2  S.  9  : 95-102  A.  p.  280. 

3  Chapters  V  and  XII;  also  G.  Chapter  XXXIX;  S.  7  :  335-345;  A.  p.  950. 


36 


ELECTRICAL  EQUIPMENT 


kw.  either  motor  may  be  used,  depending  on  conditions  under 
which  it  is  used.  Diagrams  of  the  connections  of  the  two  kinds 
of  motor-generator  sets  are  given  in  Fig.  10. 

(6)  A  synchronous  converter  has  advantages  over  a  motor-genera- 
tor set,  in  that  the  cost  is  less  and  the  efficiency  higher,  especially 
if  transformers  are  used  in  both  cases.1  It  has  the  disadvantages 
of  inflexibility:  the  ratio  of  D.C.  to  A.C.  voltage  is  nearly  con- 
stant (see  Chapter  III),  and  the  power  factor  cannot  be  varied 
through  any  considerable  range.2  To  overcome  these  difficulties, 
the  synchronous  booster-converter  has  been  developed.3  It  con- 


D  C.        From 
"*7      3  Phase 

0utPut      Source 


Converter       Booster 


W 

FIG.  11. — Synchronous  converter  with  and  without  a  booster. 

(a)  Three-phase  synchronous  converter.  With  this  connection  it  can  be  started  from  the 
D.C.  end.  Special  field  connections  are  necessary  for  self-starting  from  the  A.C.  end.  (6) 
Three-phase  synchronous  booster  converter,  showing  A.C.  connections  from  the  three-phase 
source  through  the  booster,  to  the  converter.  The  rheostat  can  be  manipulated  so  that 
the  field  current  flows  in  either  direction,  and  the  boosting  is  either  positive  or  negative. 

sists  of  a  synchronous  converter  direct-connected  to  an  A.C. 
generator  which  is  used  as  a  booster.  This  Combination  costs 
somewhat  more  than  the  synchronous  converter  alone,  but  less 
than  the  motor-generator  set.  Its  efficiency  is  nearly  as  high  as 
that  of  the  synchronous  converter,  and  it  has  the  voltage  flexi- 
bility of  the  motor-generator  set.  The  external  connections  of 
these  machines  and  some  of  the  internal  connections  of  a  syn- 
chronous booster-converter  are  shown  in  Fig.  11. 

1  For  data,  applications  and  operation  see  G.  Chapter  XXXIX;  S.  9  : 
38-84;  A.  pp.  279,  280,  290,  291. 

2  A.  p.  950:  Induction  motor  driving  D.C.  generator. 

p.  279:  Synchronous  converter  versus  motor-generator. 
S.   9:  55-61    Comparison   of   motor-generator   sets   and   synchronous 
converters. 

7:  332  Flexibility  of  motor-generator. 
12:  63  Efficiencies,  costs  and  floor  space. 

3  S.  9 :  20  The  synchronous  booster-converter. 


CONVERTERS 


37 


(c)  A  mercury  vapor  rectifier  consists  essentially  of  a  receptacle 
containing  mercury  vapor,  connected  to  a  single-phase  or  poly- 
phase source,  from  which  the  mercury  vapor  causes  the  selection 
of  a  current  flowing  in  only  one  direction.1     Its 

best  application  at  the  present  time  is  to  circuits 
of  110  volts  or  more,  and  relatively  small  cur- 
rents. Rectifiers  are  made  for  use  in  connection 
with  series  lighting  circuits,  for  as  many  as 
seventy-five  6.6  amp.  arc  lamps,  but  they  are 
more  often  made  to  be  connected  to  110-  or 
220-volt  A.C.  circuits,  for  charging  batteries  at 
D.C.  voltages  from  2  to  120,  and  currents  from 
5  to  50  amp.  A  simple  diagram  of  a  mercury 
vapor  rectifier  is  shown  in  Fig.  12. 

(d)  A  vibrating  rectifier  accomplishes  mechan- 
ically what  the  mercury  vapor  does  by  other 
means — it  has  a  vibrating  contact  that  closes,  at 
such  times  that  the  current  can  flow  in  only  one 
direction.2     This  rectifier  is  made  for  low  voltage 
only,   at  which  the  efficiency  of  the   mercury 
vapor  rectifier  is  very  low.      It  is  connected  to 
a   110-  or  220-volt  single-phase  circuit,  and  de- 
livers as  much  as  8  amp.  to  three  lead  storage  cells 

in  series.     The  external  connections  of  a  vibrating 
rectifier  may  be  represented  as  in  Fig.  13. 

While  there  is  some  overlapping  in  the  application 
of  these  converting  devices,  each  has  certain  features 
peculiar  to  itself  that  adapt  it  to  certain  kinds  of 
service:  (1)  If  a  transmission  line  has  a  large  voltage 
drop,  a  motor-generator  set  or  a  synchronous  booster- 
converter  will  probably  be  the  best  choice,  because 
D  the  D.C.  voltage  can  be  maintained  constant  with 


D.O.Output 
to  Battery 

FIG.  12.— 
Mercury  vapor 
rectifier. 

The  A.C.  source 
connects  through  an 
a  u  t  o-transf  ormer 
to  the  rectifier. 
The  zig-zag  line  in 
the  lower  right- 
hand  corner  of  the 
diagram  represents 
a  resistance  leading 
to  a  starting  ter- 
minal. 


to  Battery 

FIG.  13.— 
Vibrating 
rectifier. 


fluctuating  A.C.  terminal  voltage.     If  the  power  factor 
of    the  line  current  is  low,   the  motor  field  of  the 
synchronous  motor-generator  set  can  be  over-excited, 
to  produce  a  leading  current,  and  raise  the  power  factor.     (2) 
Where  line  drop  is  low  and  power  factor  high,  the  ordinary 
synchronous  converter  can  be  used  to  advantage,  because  it 

1  G.  388;  S.  6  : 269-281;  A.  pp.  1209,  1210. 
2S.  6:289-295;  A.  p.  1211. 


38 


ELECTRICAL  EQUIPMENT 


is  more  efficient  and  a  little  less  expensive,  especially  if  it  re- 
quires no  additional  transformers.  (3)  For  smaller  currents,  the 
rectifiers  have  their  field  of  usefulness,  the  mercury  vapor  rectifier 
being  suited  to  all  voltages  above  the  8  or  10  volts  required  for 
charging  three  lead  storage  cells.  Table  VI.  shows  the  efficiency 
of  the  various  means  for  converting,  under  the  conditions  for 
which  they  are  adapted. 

TABLE.  VI. — COMPARISON  OP  EFFICIENCIES  OF  THE  SEVERAL  KINDS  OF 
CONVERTING  AND  RECTIFYING  APPARATUS. 


Apparatus 

Kw. 

Amperes  in 
D.C.  circuit 

D.  C.  voltage 

Approximate 
efficiency  at 
full-load 
(per  cent.) 

Motor  generator 

1     2  to  10 

70 

set 

/  15  to  300 

80 

Synchronous 

converter  or 
synchronous 
booster  con- 

2 to  10 
15  to  300 

85 
90 

verter  

Mercury    vapor 
rectifier,     con- 
stant potential 

} 

5  to  50 
5  to  50 
5  to  50 
5  to  50 

15 
50 

no 

220 

50 
75 

85 
90 

Mercury    vapor 

rectifier 
For  constant 
current 

4 
6.6 

Up  to  5500 
Up  to  4000 

Up  to  95 
Up  to  95 

circuits  .  .  . 

J 

Vibrating 
rectifier 

} 

8 

5  to  10 

55 

7.  Raising  or  Lowering  D.C.  Voltage. — There  are  several  con- 
ditions requiring  a  change  of  D.C.  voltage.  Those  most  fre- 
quently found  in  practice,  on  constant  potential  circuits,  and  the 
means  employed  in  producing  the  change  are  as  follows: 

Three-wire  System  for  Lighting  Circuit.1 — The  most  satisfactory 
voltage  for  arc  and  tungsten  lighting  is  110  volts.  This  voltage 
is  too  low  for  satisfactory  distribution  in  a  large  industrial  plant, 
in  which  the  motors  are  usually  operated  on  220  volts.  The 


G.  368;  S.  13:82,  83;  A.  p.  366. 


CONVERTERS 


39 


generators  furnish  power  at  the  motor  voltage,  and  some  means 
is  to  be  provided  for  furnishing  the  lower  voltage  to  the  lamps. 
If  the  lamps  are  adapted  to  one-half  the  motor  voltage,  they  may 
be  connected  from  one  of  the  lines  to  neutral.  A  balancer-set 
may  be  installed  as  indicated  in  Fig.  14,  to  keep  this  voltage 
actually  neutral — that  is,  midway  between  positive  and  negative.1 
This  set  consists  of  two  shunt  or  compound  machines  that  are 
exactly  alike,  direct-connected  mechanically,  and  connected 
electrically  in  series  across  the  outside  lines.  The  neutral  line 
connects  to  a  point  between  the  two  armatures. 


From  Generators 


Balancer  Set  2  &  3  Wire  Li8htiug  Feeders, 
To  Motor  « 


„  To 
.Motor 


To  Motor 
Power  Feeder 

FIG.  14. — Balancer  set  on  D.C.  3-wire  system. 

When  the  lamp  load  is  exactly  balanced — that  is,  when  the 
current  flowing  from  the  positive  line  is  the  same  as  that  flowing 
to  the  negative — there  is  no  current  flowing  back  in  the  neutral 
wire,  and  the  two  machines  of  the  balancer-set  float  on  the  line 
as  idle  motors.  But  if  more  or  larger  lamps  are  on  one  side  of  the 
line  than  on  the  other,  the  joint  resistance  of  lamps  on  that  side 
is  less.  As  a  result,  the  voltage  across  that  side  would  drop, 
and  that  across  the  other  side  would  increase,  except  for  the 
balancer-set,  which  keeps  the  neutral  wire  at  or  near  actual 
neutral  voltage,  allowing  the  unbalanced  portion  of  the  current 
to  return  through  the  neutral  and  the  balancer-set,  instead  of 
flowing  through  the  lamps.  With  this  unbalanced  condition  of 
the  load,  the  machines  are  running,  one  as  a  generator  furnishing 
the  unbalanced  current  to  the  side  requiring  more;  and  the  other 
as  a  motor  driving  the  generator.  If  each  of  the  machines  has 
a  series  winding,  the  connections  can  be  made  as  shown,  so  that 
the  current  of  the  neutral  wire  flows  in  such  directions  through  the 
series  fields  as  to  decrease  the  voltage  on  one  side  and  to  increase 

1  G.  347;  S.  8:  224,  225;  A.  p.  375. 


40 


ELECTRICAL  EQUIPMENT 


FIG.  15. — Three-wire  D.C. 
generator  and  balance  coils. 


it  on  the  other,  compensating  for  RI  drop  in  the  neutral  line  and 
armature. 

Instead  of  the  balancer-set,  a  three-wire  generator  may  be 
employed,  as  in  Fig.  15. x  The  generator  itself  is  essentially  a  two- 
phase  synchronous  converter.  Each  balance  coil  is  simply  a 
coil  of  large  reactance  connected  by  slip-rings  across  one  of  the 
phases.  On  account  of  the  high  reactance,  the  alternating 
current  flowing  through  the  coil  from  one  slip  ring  to  the  other 
is  negligibly  small.  The  middle  points  of  the  two  coils  are  at 

neutral  voltage;  they  are  connected 
together,  and  to  the  neutral  line. 
Since  direct  current  is  not  affected 
by  reactance,  it  can  flow  readily  in 
the  neutral  line,  either  to  or  from  the 
generator.  Sometimes  only  a -single 
balance  coil  is  used,  and  is  connected 
to  single-phase  slip-rings.  In  Fig.  15 
the  slip-rings  and  the  commutator  are 
shown  at  opposite  ends  of  the  arma- 
ture. Frequently  the  two  are  at  the  same  end,  but  the  opera- 
tion is  unchanged. 

Multiple  Voltage  for  Motor-speed  Adjustment.2 — A  three- wire 
circuit  such  as  has  just  been  described  for  lighting  purposes  can 
be  used  for  motor-speed  adjustment.  When  the  armature  is 
connected  from  one  line  to  neutral  the  speed  is  practically  one- 
half  that  when  it  is  across  the  outside  lines.  Further  speed  con- 
trol is  obtained  by  a  field  rheostat,  as  stated  in  Chapter  IV,  page  25. 
Ward  Leonard  and  Ilgner  Systems  for  Motor-speed  Adjustment.3 
—The  Ward  Leonard  system  may  be  considered  as  a  refinement 
of  the  multiple- voltage  system.  It  is  limited  to  the  few  cases 
where  the  fine  adjustment  obtained  is  worth  the  cost.  The  motor 
whose  speed  is  to  be  adjusted  has  its  field  excited  from  a  con- 
stant-potential source,  and  its  armature  is  connected  to  the 
generator  armature  of  a  motor-generator  set.  Thus  in  Fig.  16, 
motor  A,  which  is  connected  to  the  constant-potential  source, 
drives  generator  B  at  constant  speed.  Rheostat  R  controls 
the  voltage  of  the  generator.  Motor  C,  whose  speed  is  to  be 
adjusted,  has  a  constant  current  in  its  field,  and  a  variable 

1  G.  348;  S.  8:  190-199. 

2  G.  129;  S.  15:  448;  A.  p.  966. 

3  G.  130;  S.  7:  341-345;  S.  15:99-107;  A.  p.  976. 


CONVERTERS 


41 


voltage  across  the  armature,  depending  on  how  much  of  the 
resistance  of  rheostat  R  is  in  the  circuit.  The  motor  speed 
depends  on  its  armature  voltage  and  is  thus  controlled  by  the 
generator  field  rheostat.  The  direction  of  rotation  of  the  motor 
is  reversed  by  reversing  the  field  connections  to  the  generator. 
Sometimes  motor  A  is  an  induction  motor,  but  motor  C  and 
generator  B  are  always  D.C.  machines. 


Generator 


Motor 


A  B  C 

FIG.  16. — Ward  Leonard  system  of  motor  speed  control. 

Motor  A  is  sometimes  an  A.C.  motor.     The  Ilgner  system  is  similar  to  this,  having 
fly-wheel,  as  shown  dotted,  driven  by  motor  A,  which  is  usually  an  A.C.  motor. 


The  Ilgner  system  is  similar  to  the  Ward  Leonard;  in  addition 
there  is  put  on  the  shaft  of  the  motor-generator  set  a  flywheel 
that  gives  up  its  energy  when  the  motor-generator  set  slows 
down.  This  system  is  used  for  hoisting,  and  is  subject  to  short, 
heavy  peak  loads.  As  usually  installed,  motor  -A  is  a  slip-ring 
induction  motor,  and  B  and  C  are  D.C.  machines.  The  speed 
of  motor  A  is  controlled  by  an  automatic  device  to  utilize  the 
energy  in  the  flywheel  when  the  peak  loads  come;  and  the  speed 
of  hoisting  by  motor  C  is  controlled  by  the  generator  field 
rheostat. 

Boosters  for  Battery  Control,  and  Line-drop  Compensation.1 — A 
booster  is  merely  a  generator  used  to  raise  or  lower  the  line  or 
other  voltage  when  it  is  necessary.  It  is  used  to  raise  or  lower 
the  voltage  of  a  battery  circuit,  so  as  to  make  the  battery  charge 
or  discharge  more  strongly  than  it  would  if  it  were  floating  on  the 
line  without  the  booster.  A  booster  is  also  used  to  raise  the 
voltage  on  a  long  feeder,  in  which  the  voltage  drop  would 
otherwise  be  excessive. 

i  G.  204-209,  346;  S.  8:  184;  S.  12:86;  S.  20:147-160,  165,  166;  A.  pp. 
97-100. 


42  ELECTRICAL  EQUIPMENT 

The  voltage  of  the  booster,  which  is  superposed  on  the  battery 
or  line  voltage,  may  be  regulated  either  by  hand  or  automatically. 
If  it  is  regulated  by  hand,  the  field  circuit  is  connected  through 
a  field  rheostat,  across  the  line.  The  machine  is  then  called  a 
shunt  booster.  If  the  operation  is  to  be  automatic,  at  least  one 
winding  of  the  booster  is  in  series  with  the  circuit  that  controls 
the  booster  voltage.  For  example,  for  compensating  for  line 
drop  the  field  coil  of  the  booster  is  in  series  with  the  line ;  and  for 
maintaining  approximately  constant  current  in  the  generator, 
a  booster  field  winding  is  in  series  with  the  generator.  There 
are  a  number  of  ingenious  arrangements  used  in  connection  with 
battery  charging  and  discharging,  that  make  the  battery  take 
a  part  or  practically  all  the  fluctuation  of  the  load  on  the  line, 
so  that  the  load  on  the  generator  can  be  made  to  remain  prac- 
tically constant. 


110  or  220  Volt 
Source 


FIG.  17. — Dynamotor  for  delivering  very  low  voltage. 

Very  Low  D.C.  Voltage. — A  dynamotor1  is  especially  suited  for 
supplying  a  heavy  current  at  a  very  few  volts;  for  example,  for 
some  electrolytic  work.  As  shown  in  Fig.  17,  a  commutator  at 
one  end  connects  to  the  higher  D.C.  voltage — usually  110  or  220 
volts.  This  commutator  and  the  corresponding  winding  serve 
as  a  motor  element,  driving  the  machine  at  constant  speed. 
Another  commutator  at  the  other  end,  and  its  low-voltage  wind- 
ing, serve  as  the  generator  element.  The  high-  and  low-voltage 
windings  are  entirely  distinct,  but  are  laid  in  the  same  slots  and 
revolve  in  the  same  field. 

1  S.  9:  126-133;  A.  p.  382. 


CHAPTER  VII 
TRANSFORMERS1  AND  AUTO -TRANSFORMERS 

The  original  use  of  transformers  was  to  step  the  A.C.  voltage 
up  at  the  beginning,  and  down  at  the  end  of  a  transmission  line. 
Since  the  original  applications,  new  applications  of  that  type  of 
transformer  have  been  made,  and  various  other  kinds  of  trans- 
formers have  been  applied  to  new  uses.  We  shall  consider  several 
of  the  more  important  applications,  beginning  with  the  original 
step-up  and  step-down  transformers. 

Applications  and  Operation  of  Transformers. — Transformers 
for  stepping  up  the  voltage  are  unnecessary  if  the  generator 
voltage  is  high  enough;  but  it  pays  to  build  small  generators  for 
relatively  low  voltage  and  to  step  the  voltage  up,  rather  than  to 
build  small  generators  for  very  high  voltage. 

Voltage  and  Ratio. — A  transformer  should  not  be  put  across  a 
voltage  much  above  its  rated  voltage,  because  if  the  amount  of 


00  W 

FIG.  18. — Transformer  with  the  secondary  winding  in  two  equal  sections. 
In  (a)  the  sections  are  in  parallel,  and  in  (Z>)  in  series.     Sometimes  the  primary  winding 
is  also  in  sections,  which  may  be  either  in  parallel  or  in  series. 

iron  has  not  been  generous,  the  resulting  magnetizing  current 
may  be  excessive.  Too  high  voltage  also  endangers  the  insula- 
tion; but  the  magnetizing  current,  rather  than  the  insulation, 
usually  limits  the  voltage.  If  the  line  voltage  is  lower  than  the 
transformer  rated  voltage,  no  harm  is  done  except  that  in  ex- 
treme cases  the  voltage  regulation  and  efficiency  are  poor,  and 
the  kva.  capacity  goes  down  with  voltage. 

1  G.  Chapter  XXXIV,  Characteristics;  XXXV,  Connections. 

S.  6:85-126;  133-135;  137-145;  147-149;  155-161,  Characteristics,  data 
and  connections.    S.  10:  837-849,  Applications. 

A.  pp.  1606-1610,  Classification  and  Theory.     1612-1617,  Connections. 
1632-1637,  Applications,  weights  and  costs. 

See  Chapters  XIV  and  XV,  Regulating  and  Instrument  Transformers. 

43 


44  ELECTRICAL  EQUIPMENT 

To  guard  against  applying  to  wrong  voltages,  it  is  well  to  ex- 
press the  ratio  in  actual  volts,  as  2,200/110,  rather  than  to 
make  the  numerator  or  the  denominator  unity,  as  20/1  or  1/20. 
If  a  2,200-volt  transformer  be  put  on  a  2,500-volt  circuit  the 
operator  will  then  know  that  the  magnetizing  current  will  be 
excessive. 

Some  transformers  are  provided  with  voltage  adjustment, 
which  is  in  one  or  more  of  four  ways:     (a)  The  secondary  is 
divided  into  two  or  more  equal  parts,  which  are 
to  be  connected  either  in  parallel,  as  in  Fig.  18a, 
or  in  series,  as  in  Fig.  186,  depending  on  the 

FIG.  19.—  required  secondary  voltage,  (b)  Several  leads 
Transformer  with  ,  ,  ,  Jf  .  ,  ' 

several  secondary     are  brought  out  from  points  a  few  per  cent. 

leads  brought  out     from  one  end  of  the  secondary  winding,  as  in 
for  adjustment  of      ,.,.       „_       „,.  ,,  A,      ,       <. .-,.      .,„        , 

voltage.  Fig.  19.     Of  course  the  methods  of  Fig.  18  and 

Similar  leads  are  Fig.  1Q  cannot  be  Combined,  UnleSS  both  Wind- 
sometimes  brought  .  . 

out  from  the  pri-  mgs  in  Fig.  18  are  provided  with  additional 
leads  shown  in  Fig.  19,  (c)  The  primary  is 
divided  into  equal  parts,  as  the  secondary  is  divided  in  Fig.  18. 
(d)  Primary  leads  are  brought  out  as  in  Fig.  19. 

The  application  of  transformers  with  extra  leads,  such  as  the 
foregoing,  must  be  with  due  care.  Unless  there  is  definite  infor- 
mation to  the  contrary,  the  transformer  should  not  be  put  on  a 
line  of  the  highest  rated  primary  voltage,  unless  the  primaries 
are  in  series,  and  all  or  nearly  all  the  end  turns  are  in  circuit; 
it  should  be  assumed  that  primary  end  turns  are  intended  for 
adjusting  for  low  primary  voltage. 

Capacity  in  Kva. — The  product  of  the  voltage  times  the  cur- 
rent in  a  single-phase  circuit  is  called  the  apparent  power,  and  is 
expressed  in  volt-amperes. 1  This  is  the  same  as  the  power  in  watts, 
if  the  power  factor  is  at  100  per  cent.,  but  at  lower  power  factors 
the  number  of  volt-amperes  is  greater  than  the  number  of  watts. 
This  unit  and  the  kilovolt-ampere  (=  1,000  volt-amp.)  are 
used  to  designate  the  capacity  of  A.  C.  apparatus  to  trans- 
form or  deliver  current  at  a  given  voltage.  Transformers 
and  A.C.  generators  are  regularly  rated  in  kilovolt-amperes 
(abbreviated  to  kva.),  rather  than  in  kilowatts,  because  a  kilo- 
watt rating  has  no  definite  significance  unless  the  power  factor 
is  given.  If  the  current  is  balanced  on  a  polyphase  circuit,  the 

'G.  246;  S.  24:27;  A.  p.  1298. 


TRANSFORMERS  AND  AUTO-TRANSFORMERS     45 

same  relation  exists  as  on  single-phase,  between  watts  and 
volt-amperes.  Thus,  we  have, 

On  a  single-phase  circuit  kva.  =  kw./P.F.  =  #7/1,000 
On  a  two-phase  circuit  kva.  =  kw./P.F.  =  2#//l,000 
On  a  three-phase  circuit  kva.  =  kw./P.F.  =  \/3#//l,000 

where  E  is  the  voltage  between  lines  and  I  is  the  current  per 
line  leading  to  the  transformer  or  group  of  transformers. 
It  is  at  once  evident  that  to  get  the  maximum  power  from  trans- 
formers and  other  similar  equipment,  they  should  deliver  power 
at  as  high  a  power  factor  as  possible. 

If  a  transformer  is  used  only  intermittently,  it  will  deliver 
safely  much  more  than  the  rated  kva.  output,  for  the  short  time 
(see  Chapter  XX,  p.  183). 


0.333 


0        5       10      15      20      25      30       35      40      45       50      55      60      65      70       75     80 
Bated  Amperes  in  High.  Tension  Winding 

FIG.  20. — Curves  showing  full-load  efficiency  and  regulation  of  typical 

transformers. 


Efficiency. — Fig.  20  shows  how  transformer  full-load  efficiency 
depends  on  the  current  capacity  of  the  high-tension  winding. 
These  curves  are  based  on  data  on  about  100  typical  trans- 
formers of  voltages  from  2,200  to  110,000,  and  are  correct  in  most 
cases  within  0.1  per  cent,  or  0.2  per  cent.  It  will  be  seen  that  the 
losses  in  transformers  made  for  25  cycles  are  nearly  1.5  times  those 
for  60  cycles.  The  variation  of  efficiency  with  load  in  a  typical 


46 


ELECTRICAL  EQUIPMENT 


transformer  is  illustrated  in  Fig.  21.  In  some  transformers  full- 
load  is  a  little  nearer  the  point  of  maximum  efficiency  than  is  in- 
dicated on  this  curve. 

Regulation. — The  per  cent,  regulation  is  the  per  cent,  ratio  of 
the  change  in  secondary  voltage  between  no-load  and  full-load 
to  the  transformer  rated  secondary  voltage.1  Fig.  20  shows 
the  regulation,  as  well  as  the  efficiencies  of  typical  transformers, 
when  the  load  is  at  100  per  cent,  power  factor.  At  other  power 
factors  the  regulation  is  poorer  than  at  100  per  cent.  In  a 
transformer  having  a  high-tension  voltage  of  13,200  or  less,  with 


100 
99.5 


r 

«£   97.5 
97 


0   10   20   30   40   50   60   70   80   90   100  110  120  130 
Percent  of  Full  Load 

FIG.  21. — Curve  showing  variation  of  efficiency  with  load  in  a  typical 
power  or  lighting  transformer. 

a  lagging  current,  the  regulation  at  80  per  cent,  power  factor  is 
usually  between  2  and  3  per  cent.  With  a  leading  current  at 
80  per  cent,  power  factor,  the  regulation  is  not  far  from  zero. 

Frequency. — The  weight  and  cost  of  a  transformer  are  greater 
for  low  than  for  high  frequencies  (see  Chapter  III,  p.  17).  A 
transformer  designed  for  a  given  frequency  can  be  used  on  a 
circuit  of  a  higher,  but  not  of  a  lower  frequency,  at  the  same 
voltage. 

An  auto -transformer2  is  a  transformer  in  which  the  primary 
and  secondary  are  combined  in  a  single  circuit,  as  in  Fig.  22a, 
where  the  primary  is  connected  across  AB  and  the  secondary 
across  AC,  or  vice  versa.  The  winding  from  A  to  C  is  common 

!S.  24:560,  565;  A.  pp.  1327,  1328. 
2  G.  307,  332,  333. 

8.6:173-178,  180,  181. 

A.  pp.  63-65. 


TRANSFORMERS  AND  AUTO-TRANSFORMERS     47 

to  both  primary  and  secondary  and  that  from  C  to  B  is  in  only 
the  circuit  of  the  higher  voltage — which  we  will  call  the  primary. 
The  advantage  of  an  auto-transformer  over  a  transformer  is 
seen  by  comparing  Fig.  22a  with  6.  Neglecting  all  losses  and 
the  reactance  drop,  the  apparent  watts  input  equal  the  output. 
If  the  avto -transformer  is  used  to  step  down  from  110  to  100 
volts,  and  is  to  deliver  11  amp.,  the  current  taken  from  the 
primary  line  must  be  10  amp.  Winding  CB  must  carry  10  amp., 
and  winding  CA  must  furnish  the  other  1  amp.  Thus,  winding 
CB  is  for  10  volts  X  10  amp.,  or  100  volt-amp.,  and  winding  CA 


Secondary 


Primary 


Secondary 


Primary 


FIG.  22. — Auto-transformer  and  transformer,  with  primary  voltage 
higher  than  the  secondary. 

If  the  contact  at  C  is  movable,  and  leads  are  brought  out  from  suitable  points  in  the 
winding,  any  desired  secondary  voltage  can  be  obtained,  between  the  primary  voltage  and 
zero.  If  the  primary  and  secondary  are  interchanged,  the  secondary  voltage  is  higher  than 
the  primary.  Such  a  connection  must  be  made  with  care,  to  avoid  too  high  a  voltage  for 
the  shorter  winding. 


is  for  100  volts  X  1  amp.  or  100  volt-amp.  But  each  winding 
of  a  transformer,  as  in  Fig.  226,  must  be  for  1,100  volt-amp.; 
so  that  the  required  size  of  the  auto-transformer  is  only  one- 
eleventh  that  of  the  transformer  to  do  the  same  work. 

One  can  readily  see  also  that  the  efficiency  of  transformation 
is  much  higher  in  the  auto-transformer  than  in  the  transformer, 
because  the  losses  are  very  small — in  the  case  above,  correspond- 
ing to  the  losses  in  a  transformer  of  one-eleventh  the  size  that 
would  be  required  for  ordinary  transformation. 

The  advantage  in  size  and  efficiency  is  less  marked,  when  the 
ratio  is  farther  from  unity.  In  the  case  of  Fig.  22,  with  the  same 
primary  voltage,  if  the  output  were  at  11  volts,  instead  of  100, 
the  volt-ampere  capacity  of  the  auto-transformer  windings  would 
be  nine-tenths  of  that  of  the  corresponding  transformer. 

One  of  the  most  important  applications  of  auto-transformers 
is  to  starting  squirrel-cage  induction  motors.  This  is  better 
than  to  use  rheostats,  because  less  power  is  wasted.  It  is  better 


48  ELECTRICAL  EQUIPMENT 

than  to  use  ordinary  transformers,  because  the  auto-transformers 
are  smaller  and  less  expensive. 

3.  Grouping  of  Transformers. — The  only  groupings  of  trans- 
formers that  we  shall  consider  are  some  of  those  on  three-phase 
circuits.  Two-phase  combinations  are  omitted  because  they 
are  comparatively  simple,  and  require  no  explanation  except 
what  is  given  in  this  chapter  with  reference  to  the  Scott  connec- 
tion. Six-phase  connections  are  important  for  those  who  have 
to  do  with  large  synchronous-converter  substations;  but  they  are 
little  used  elsewhere.  , 

Several  of  the  more  usual  groupings  of  transformers  are  illus- 
trated in  Fig.  23.  Of  these,  the  delta  connection,  Fig.  23a,  is 
used  more  than  any  other,1  for  ordinary  transformation. 

Perhaps  the  greatest  advantage  of  having  a  delta  connection 
in  both  primary  and  secondary  is  that  in  case  of  breakdown  of 
one  transformer,  it  can  be  removed,  and  the  group  can  continue 
to  operate  with  a  V-connection.  The  two  remaining  transformers 
will  be  carrying  a  73  per  cent,  overload,  if  they  deliver  the  same 
power  as  was  required  of  the  three  at  full-load,  but  if  the  load  is 
cut  down  to  58  per  cent,  of  the  original  full-load,  the  two  are 
only  carrying  normal  load.  This  fact  holds  true,  not  only  for  a 
group  of  three  single-phase  transformers,  but  also  for  a  three- 
phase  transformer,  if  one  phase  is  out  of  commission.  In  a 
three-phase  core-type  transformer  care  must  be  observed  that 
there  are  no  dangerous  loose  ends  in  the  broken-down  phase  of 
the  transformer;  for  there  is  full  voltage  in  all  phases,  even 
though  both  primary  and  secondary  of  one  phase  are  entirely 
disconnected  from  the  system.  The  voltages  are  not  as  well 
balanced,  and  the  efficiency  of  transformation  is  lower  with  the 
V  than  with  the  delta  connection. 

The  Scott  connection  is  illustrated  in  Fig.  23/i.  Two  trans- 
formers are  T-connected  to  a  three-phase  circuit;  and  the  other 
windings  of  the  transformers  are  connected  to  a  two-phase  circuit. 
This  connection  is  suitable  for  transforming  either  from  three-  to 
two-phase  or  from  two-  to  three-phase. 

To  appreciate  the  importance  of  this  somewhat  familiar  con- 
nection, imagine  a  machine  shop  in  a  city  such  as  Philadelphia 
or  New  York,  in  which  two-phase  power  has  been  obtainable  for 

1  Except  on  very  high  voltages  where  the  Y-connection  is  used  on  the 
high-tension  side;  the  low-tension  side  may  even  then  be  delta-connected. 
See  Fig.  236  and  d. 


TRANSFORMERS  AND  AUTO-TRANSFORMERS     49 

a  long  time.  Suppose  that  a  change  in  management  of  the  elec- 
tric power  company  results  in  changing  from  a  two-phase  to  a 
three-phase  system.  It  is  only  necessary  to  install  a  pair  of 
Scott-connected  transformers,  to  continue  the  two-phase  system 


Delta  to  Delta  Connection 

This  connection  is  nearly 

always  used  for  ordinary 

Power  purposes  except  on 

very  High  Voltages 


~7j      — ' — »r~ 

^  A 


(bw± 

Neutral  Line  or  Ground  on  One 
or  Both  Sides  if  Required 

Y  to  Y  Connection 


Ground  Connection,  if  Required. 

Y  to  Delta  or  Delta  to  Y 

Connection 


C«) 

V  to  V  Connection 

Used  in  Emergencies  when 
One  Transformer  in  a  Delta 
Connection  Breaks  Down. 
Used  also  with.  Voltage 
Transformers  for  Meters. 


V  Connection 
of  Auto-trans- 
formers for 

Motor 
Starting 


This  Transformer  is 
Preferably  Wound.for  only 
86.6  %  of  Line  Voltage 
on  each  Side 


50%    Tap  on  each 

e  of  this  Transformer 


00 

T  to  T  Connection 


>  86.6  %  Tap,  or   Winding  for 
-*       86.6  %    of  Line  Voltage 


T  to  2 -Phase  or 
Scott  Connection 


FIG.  23.  —Transformer  grouping. 


w 

Scott  Connection 
of  Auto-transformers 


in  the  shop;  whereas  without  this  connection  or  its  equivalent, 
it  would  be  necessary  to  take  out  every  two-phase  motor  in  the 
shop  and  replace  it  by  a  three-phase  motor,  costing  from  $100  to 
$1,000  for  every  motor  changed. 


50  ELECTRICAL  EQUIPMENT 

The  Scott  connection  may  be  employed  to  step  the  voltage  up 
or  down,  or  to  transform  with  no  change  in  voltage.  If  the  two 
voltages  are  the  same,  or  nearly  the  same,  auto-transformers 
may  be  used;  there  must  then  be  no  other  electrical  connection 
between  the  two  phases  of  the  two-phase  circuit.  The  auto- 
transformer  connections  for  equal  two-phase  and  three-phase 
voltages  are  shown  in  Fig.  232. 


CHAPTER  VIII 

STORAGE  BATTERIES1 

COMPARISON  OF  TYPES  OF  STORAGE  BATTERIES 

The  storage  cells  used  in  practice  are  of  two  kinds.  One  of 
these  is  the  lead  cell,  whose  electrodes  are  lead  and  lead  peroxide, 
and  whose  electrolyte  is  dilute  sulphuric  acid.  One  form  of  lead 
cell  has  "pasted"  plates,  in  which  the  active  material  is  added 
in  the  form  of  a  paste.  Another  has  "Plante"  plates  which  are 
formed  electrochemically  by  putting  them  in  the  electrolyte  and 
passing  a  current  repeatedly  first  in  one  direction  and  then  in 
the  other.  A  third  form  of  lead  cell  is  the  " Ironclad,"  whose  posi- 
tive plates  are  made  up  of  a  series  of  hard-rubber  slotted  tubes 
containing  the  active  material.  Besides  these  lead  cells  are  the 
alkaline  cells.  The  Edison  storage  battery  is  of  this  type,  and  is 
the  alkaline  battery  in  most  general  use;  it  has  caustic  potash 
for  the  electrolyte,  and  nickel  peroxide  and  iron  for  the  electrodes. 
So  much  depends  on  the  size  and  kind  of  installation,  and  the 
treatment  the  battery  receives,  that  the  following  information 
must  be  taken  only  as  a  general  guide,  subject  to  large  correc- 
tions in  individual  cases.  Among  the  more  important  considera- 
tions are  the  following: 

Cost. — Alkaline  batteries  cost  about  $80  per  kw.-hr.  capacity; 
portable  lead  batteries  with  pasted  plates  cost  $30  to  $55  per 
kw.-hr.  capacity.  The  cost  of  stationary  batteries  with  Plante 
plates  is  1.5  to  2  times  that  for  pasted  plates,  depending  on  size, 
liberality  of  construction  and  various  other  details.  It  is  not 
enough  higher  to  warrant  the  use  of  pasted  plates  where  the  more 

1 G.  Chapter  XXIII,  Characteristics;  Chapter  XXV,  Applications; 
Chapter  XXVI,  Train  Lighting. 

S.  20:  43-233,  Characteristics  and  Applications;  10:  898,  900  Deprecia- 
tion; 12:  77-79  Stationary  Installations;  17:  62-84;  22:  67-85  Vehicle  Bat- 
teries and  Charging;  22:  292-298  Train  Lighting  Systems. 

A.  Lead  Batteries  pp.  103-119;  Alkaline  Batteries  pp.  77-86;  Applica- 
tions pp.  87-102. 

51 


52  ELECTRICAL  EQUIPMENT 

durable  Plante  plates  can  be  put  in  small  enough  compass, 
except  for  "stand-by"  batteries,  and  others  used  infrequently. 

Space  Occupied  and  Weight. — The  net  space  occupied  by  an 
Edison  battery  is  about  0.7  cu.  ft.  per  kw.-hr.  of  energy  to  be 
stored.  That  of  a  portable  lead  cell  with  pasted  plates  is  from 
0.5  to  0.7  cu.  ft.,  and  that  of  a  lead  cell  with  Plante  plates  about 
1.5  cu.  ft.  per  kw.-hr.  The  weight  of  the  Edison  battery  is 
about  75  Ib.  per  kw.-hr.,  the  lead  battery  with  pasted  plates  65 
to  125  Ib.  and  with  Plante  plates  about  200  Ib.  The  volume  and 
weight  of  lead  cells  varies  considerably  depending  on  liberality 
of  design,  kilowatt-hour  capacity,  and  kind  of  retainer,  as  well 
as  on  kind  of  plates. 

Durability  and  Repairs. — The  Edison  battery  is  thoroughly 
guaranteed  by  the  manufacturers,  for  a  length  of  time  depending 
on  the  conditions  under  which  it  is  to  operate.  The  time  of  this 
guarantee  is  usually  4  or  5  years.  Lead  cells  with  pasted  plates 
'for  portable  service  are  much  less  rugged:  their  life  depends  on 
how  much  their  durability  has  been  sacrificed  to  make  them 
light,  as  well  as  on  the  kind  of  treatment  they  have.  With  rea- 
sonable care  they  can  be  fully  charged  and  discharged  from  300 
to  500  times.  Cells  with  Plante  plates  for  stationary  service, 
on  account  of  more  rugged  construction,  more  liberal  design, 
and  more  favorable  conditions  of  operation,  last  with  good  care 
from  5  to  10  years.  Pasted  plates,  under  similar  conditions, 
need  not  be  very  much  shorter  lived.  Plante"  plates  are  used  in 
train  lighting,  which  is  between  the  portable  and  stationary  in- 
stallations as  to  favorable  conditions.  Their  life  is  about  40  per 
cent,  of  that  of  the  stationary  batteries. 

Annual  cost  for  maintenance  and  repairs  of  portable  batteries 
(not  including  cost  of  charging)  is  $10  to  $20  per  kw-hr.  capacity 
for  Edison  batteries  and  about  $25  for  lead  cells  with  pasted 
plates.  Train-lighting  batteries  are  so  well  constructed  that 
maintenance  and  repairs  need  not  be  over  10  to  20  per  cent, 
of  that  of  other  portable  batteries.  Stationary  batteries,  well- 
installed  and  properly  cared  for,  require  no  allowance  for  main- 
tenance and  repairs  that  cannot  be  included  in  ordinary  attend- 
ance, together  with  the  allowance  for  replacing  them. 

The  current-discharging  rate  of  a  Plante  lead  cell  for  stationary 
use  is  usually  such  that  the  battery  discharges  in  8  hr.;  that  is, 
a  600-amp.-hr.  battery  would  normally  deliver  75  amp.  for  8 
hr.  If  such  a  battery  is  called  upon  for  more  than  75  amp., 


STORAGE  BATTERIES 


53 


the  time  is  so  much  reduced  that  the  total  ampere-hour  capacity 
is  less.  Table  VII  shows  the  maximum  time  of  discharging, 
and  the  ampere-hour  capacity,  at  various  currents. 

TABLE  VII. — AMPERES  AND  AMPERE-HOURS  OF  PL  ANTE  LEAD    STORAGE 
BATTERIES  ON  HEAVY  DISCHARGE 


When  the  discharge  rate  is 

The  battery  will  deliver  that 
current  for 

That   is,  at  that  current  it  is 
capable   of   delivering    per 
cent,    of   rated   ampere-hours 

1  X  normal  amp. 

8hr. 

100 

2  X  normal  amp. 

3hr. 

75 

3  X  normal  amp. 

1  hr.  35  min. 

60 

4  X  normal  amp. 

Ihr. 

50 

6  X  normal  amp. 

30  min. 

37 

8  X  normal  amp. 

20  min. 

33 

It  should  be  noted  that  these  percentages  indicate  the  relative 
capacity  of  the  battery  at  various  currents.  They  are  not  the 
same  as  efficiency,  although  the  efficiency  does  drop  off  very 
much  with  increase  of  current. 

For  pasted  lead  cells  with  thin  plates,  suitable  for  vehicle  use, 
the  decrease  in  capacity  on  overload  is  not  so  great  as  with 
Plante  plates.  If  they  are  discharged  in  1  hr.,  their  capacity  is 
60  per  cent,  of  that  in  8  hr. ;  if  discharged  in  2  hr.  the  capacity  is 
75  per  cent,  of  that  in  8  hr.  For  other  times  of  discharge  the 
percentage  is  correspondingly  higher  than  in  the  table.  Pasted 
cells  for  portable  use  are  not  always  rated  on  an  8-hr.,  but  fre- 
quently on  a  5-  or  6-hr,  basis. 

For  Edison  cells  the  ampere-hour  capacity  is  practically  the 
same  at  all  rates  of  discharge.  Their  current  rating  is  regularly 
based  on  discharging  in  5  hr.;  i.e.,  the  ampere-hour  capacity 
divided  by  5. 

The  Current-charging  Rate. — The  most  approved  method  of 
charging  lead  cells  is  to  give  the  battery  a  " tapering  charge" — 
that  is,  starting  the  charging  at  a  high  rate  and  gradually  reduc- 
ing the  current.  The  charging  rate  starts  at  from  1  to  2  times 
the  normal  discharge  rate,  and  finishes  at  J^  to  %  of  the  normal 
discharge  rate.  The  best  charging  of  Edison  cells  is  at  a  steady 
rate,  equal  to  the  normal  discharge  rate. 

Voltage. — The  discharge  voltage  of  a  lead  cell  is  about  2  volts, 
and  that  of  an  Edison  cell  about  1.2  volts,  when  the  cells  are  dis- 
charged at  their  normal  rates.  Fig.  24  shows  how  the  voltage 


54 


ELECTRICAL  EQUIPMENT 


drops  off  during  discharge  at  normal  current,  and  also  during 
discharge  that  is  so  rapid  that  the  cell  is  discharged  in  1  hr. 
It  also  shows  the  voltage  required  for  charging  at  the  normal 
rate.  At  higher  rates  of  charging,  the  voltage  is  a  little  higher; 
but  if  the  generator  voltage  is  high  enough  for  the  final  charge 
at  the  normal  rate,  it  will  be  ample  for  the  initial  charge  at  a 


(a)  Lead  Cell      Charging  at  Normal  Discharge  Rate 
(6)      >t       "          Discharging  at  Normal  Bate 

(c)  »       "         Discharging  in  1  Hour  at  4  Times  Normal  Eate 

(d)  Edison  Cell  Charging  at  Normal  Discharge  Rate 

(e)  "       "          Discharging  at  Normal  Rate 

(/)     »»       »>          Discharging  ia  1  Hoar  at  6  Times  Normal  Eate 


12845678  9         10 

FIG.  24. — Voltages  of  storage  cells  during  charging  and  discharging. 

higher  rate.  The  lead-cell  voltages  in  Fig.  24  refer  primarily 
to  cells  of  the  Plante  type,  but  they  apply  rather  closely  to  all 
kinds  of  lead  cells,  if  the  discharge  rate  is  on  the  8-hr,  basis. 

Efficiency. — The  efficiency  of  a  storage  battery  varies  greatly 
with  the  rate  of  charging  and  discharging.  In  ordinary  service, 
portable  Edison  batteries  have  a  watt-hour  efficiency  of  40  to 
50  per  cent.,  and  portable  lead  batteries  65  to  75  per  cent.  The 


STORAGE  BATTERIES 


55 


ampere-hour  efficiencies  of  the  same  batteries  are  60  to  70  per 
cent,  for  Edison,  and  80  to  90  per  cent,  for  lead  batteries.  On 
very  heavy  loads,  the  watt-hour  efficiency  drops  even  lower  than 
the  lowest  values  given,  and  on  very  slow  discharge  rate  it  rises 
even  higher  than  the  high  values. 


APPLICATIONS  TO  STATIONARY  SERVICE 

Since  space  and  weight  are  not  of  vital  importance,  it  is  not 
necessary  to  sacrifice  ruggedness.  The  combined  qualities  of 
durability,  high  efficiency  and  relatively  low  cost  of  the  Plante* 


-     H"H 


FIG.  25. — End-cell  switch  connected  to  storage  battery. 

cell  gives  it  first  place  in  common  use  for  stationary  service, 
although  pasted  plates  also  prove  satisfactory.  However,  if  a 
battery  is  to  be  discharged  infrequently,  even  at  very  high  rates, 
it  is  more  economical  to  install  pasted  plates,  as  their  life  com- 
pared with  that  of  Plante  plates  is  a  matter  of  discharges,  and 
not  of  time.  Several  of  the  more  common  applications  of 
batteries  may  be  mentioned: 

In  the  generating  station  a  battery  may  be  installed  in  parallel 
with  the  generators.  Its  main  purpose  is  to  relieve  the  generators 
at  certain  times,  but  it  may  also  serve  to  maintain  constant 
voltage.  Such  a  battery  may  take  the  instantaneous  fluctuations 
in  the  load  current;  or  it  may  furnish  a  part  of  the  steady  power 
output  of  the  plant  when  the  station  is  overloaded,  or  when  one 


56  ELECTRICAL  EQUIPMENT 

generator  may  thereby  be  shut  down ;  or  it  may  furnish  all  the 
power  in  an  emergency  or  when  the  load  on  the  plant  is  light 
enough  to  shut  down  all  the  generators.1 

The  internal  drop  in  the  battery  is  so  great  that  it  will  not  of 
its  own  accord  furnish  current  for  the  instantaneous  fluctuations, 
but  an  automatic  booster  in  series  with  the  battery  may  be 
excited  by  a  field  winding  in  series  with  the  generator,  in  such  a 
way  that  practically  all  the  fluctuations  in  line  current  come  from 
the  battery. 

For  taking  a  steady  load,  it  is  not  necessary  that  the  battery 
be  provided  with  an  automatic  booster.  A  shunt  booster  may 
be  used  whose  field  rheostat  is  operated  manually;  or  an  end-cell 
switch  such  as  is  illustrated  in  Fig.  25  may  be  operated  manually 
or  by  an  automatic  device,  to  make  the  battery  carry  the  fluctua- 
tions of  the  load. 

In  a  battery  substation,  if  the  voltage  drop  on  a  feeder  is 
excessive,  a  battery  may  be  connected  to  the  feeder  near  its  end, 
to  keep  the  voltage  more  nearly  constant.  A  study  of  the  battery 
characteristics  at  various  charging  and  discharging  currents  will 
show  how  great  voltage  variation  exists  when  the  battery  is 
doing  its  part  to  maintain  a  constant  voltage.  If  closer  regula- 
tion is  required,  a  booster  or  an  end-cell  switch  may  be  employed. 
The  battery  should  be  located  far  enough  out  on  the  feeder  to 
keep  up  the  voltage  at  the  end;  but  the  drop  at  intermediate 
points  should  not  be  excessive  on  account  of  having  the  battery 
too  far  out  on  the  feeder.  A  study  of  the  distribution  of  the  load, 
and  the  characteristics  of  the  battery,  will  indicate  the  most 
desirable  location. 

On  circuits  that  are  entirely  distinct,  or  connected  through 
resistance,  batteries  are  used  for  various  purposes.  If  there  are 
wide  fluctuations  of  the  voltage  on  the  power  circuit,  a  storage 
battery  may  be  first  charged  and  then  used  for  lighting,  or  for 
laboratory  or  other  purposes  requiring  a  more  constant  voltage. 
This  requires  a  separate  period  of  charging,  and  perhaps  some 
attention  to  voltage  regulation.  To  avoid  these  difficulties  the 
battery  may  be  connected  to  the  line  through  a  rheostat.  The 
higher  the  resistance  of  the  rheostat,  the  less  effect  will  fluctuations 
of  line  voltage  have  on  battery  voltage,  but  the  resistance  and  the 
number  of  cells  must  be  small  enough  to  keep  the  battery  charged. 

Besides  use  for  obtaining  very  constant  voltage,  such  a  battery 

1  For  connections  see  footnote,  p.  51. 


STORAGE  BATTERIES  57 

may  be  used  to  insure  a  voltage  when  there  is  no  generator  voltage. 
For  example,  a  circuit-breaker  should  operate  without  fail  when 
an  accident  reduces  bus  and  line  voltage  to  approximately  zero, 
but  this  is  just  the  condition  under  which  it  cannot  operate  if  it 
has  a  trip-coil  (see  Chapter  XVII,  p.  135,  Fig.  65),  operating  on 
generator  voltage.  Another  important  application  is  to  lighting 
and  other  circuits  that  require  power,  24  hr.  per  day,  where  the 
generator  runs  only  during  the  daytime. 

Still  another  important  application  is  to  apparatus  requiring 
small  amounts  of  power  at  low  voltage.  Small  motors,  bells  and 
other  small  equipment  are  sometimes  better  adapted  to  a  few 
storage  cells  than  to  higher  voltages.  Even  the  three- wire  system 
for  lighting  may  be  included  in  the  same  class.  The  neutral  volt- 
age can  be  established  by  the  middle  point  of  a  battery  that  con- 
nects across  the  entire  line. 

APPLICATIONS   TO    PORTABLE   SERVICE 

Batteries  used  for  portable  service  should  be  as  light  and  com- 
pact as  possible,  and  at  the  same  time  they  should  be  as  efficient 
and  durable  as  possible,  and  they  should  not  be  unnecessarily 
expensive.  Obviously  not  all  these  conditions  are  obtainable  to 
the  fullest  extent  in  any  one  kind  of  battery.  Referring  to  the 
advantages  of  the  various  kinds  of  batteries;  as  already  stated, 
we  find  that  an  Edison  battery  is  light  and  durable,  but  is  not 
quite  so  compact  as  the  most  compact  lead  cells,  nor  so  efficient 
as  lead  cells,  and  it  is  more  expensive.  The  several  advantages 
and  disadvantages  must  be  weighed  in  each  case,  and  those  that 
are  most  important  will  usually  dictate  the  battery  to  be  used. 
Lead  cells  with  Plante"  plates  are  little  used  for  portable  service, 
because  their  only  claim  of  great  advantage  over  the  pasted  plates 
is  in  durability;  durability  must  be  sacrificed  in  portable  batteries, 
in  favor  of  less  bulk  and  weight.  The  exception  is  in  train-light- 
ing batteries,  which  are  not  subject  to  these  restrictions  to  the 
same  extent. 

The  voltage  of  portable  batteries  is  usually  less  than  110  volts 
and  depends  on  the  amount  of  power  to  be  delivered.  There  are 
no  standards  of  voltage  that  are  now  in  universal  use,  but  the 
following  have  been  found  satisfactory  in  a  great  many  cases,  and 
are  gradually  being  adopted  as  standards: 


58  ELECTRICAL  EQUIPMENT 

Volts 

Automobile  ignition,  lighting  and  starting 6 

Electric  automobiles 60  to  85 

Battery  locomotives 80  to  240 

Battery  trucks 24  to  60 

Car  lighting  (batteries  fully  charged,  32  and  64  volts) 

nominal  voltage 30  and  60 

The  amount  of  power  required  for  each  purpose  varies  somewhat, 
but  not  greatly,  from  the  following: 

Automobile  lighting : 

Watts 

Headlights,  two  to  be  provided each,  15 

Side  lamps,  two  to  be  provided each,    3 

Tail  lamp,  one  to  be  provided each,    3 

Other  lighting,  if  desired,  need  not  exceed 15 

Gas  engine  ignition 3 

Automobile  electric  starting,  small  automobile 500 

Average  automobile 600  to  700 

Large  automobile 1,000 

Railway  car  lighting : 

Pullman  sleeper,  16  section . 1,600 

Coach 500 

Mail  car,  60-ft 650 

Baggage  car 300 

Dining  car. . . .  ; 1,600 

Automobile  Lighting,  Ignition  and  Starting  Power. — Auto- 
mobiles that  are  not  self -starting  require  very  small  battery  power 
for  lighting  and  ignition.  For  self-starting,  according  to  the 
above  table,  much  more  power  is  required,  but  even  in  that  case 
the  three-cell  battery  is  sufficient.  The  large  amount  of  power 
required  for  starting  is  used  for  only  a  very  short  time,  so  that  it 
is  allowable  to  overload  the  battery.  It  is  accepted  practice  to 
furnish  a  battery  of  such  size  that  it  will  deliver  the  required 
power  for  starting  for  a  period  of  20  min.  A  lead  battery  made  for 
this  purpose,  with  thin  pasted  plates,  will  deliver  for  20  min. 
six  or  seven  times  the  current  that  it  will  for  5  hr.,  at  an  average  of 
85  per  cent,  of  the  initial  voltage  that  it  would  have  at  normal 
load.  An  Edison  battery  will  deliver  for  20  min.  six  times  the 
current  that  it  would  for  5  hr.,  at  about  70  per  cent,  of  the  initial 
voltage  that  it  would  have  at  normal  load. 

Electric  automobiles,  battery  trucks  and  battery  locomotives 
are  driven  by  motors  operated  from  storage  batteries.1  For 

1  See Proc.  A.  I.  E.  E.,  1916,  A.  E.  Kennelly  and  0.  R.  Schurig,  "Tractive 
Resistances  to  a  Motor  Delivery  Wagon  on  Different  Roads  and  at  Different 
Speeds." 


STORAGE  BATTERIES  59 

battery  trucks,  less  power  is  required  than  for  automobiles  and 
locomotives,  and  the  customary  voltage  is  less,  as  indicated  above. 
The  resistance  to  rolling  on  a  floor  or  track  depends  on  the  kind 
of  tire,  the  kind  of  floor  or  track,  and  the  speed.  As  the  speeds 
are  not  over  12  miles  per  hr.,  if  we  have  wheels  with  solid  or 
pneumatic  tires,  the  total  resistance,  including  chain  and  bearing 
friction,  windage,  and  tire  friction,  is  about  25  Ib.  per  ton  weight 
of  truck  and  load,  on  a  good  level  floor.  With  wheels  having 
steel  tires,  on  good  rails,  the  total  resistance  of  a  locomotive  is 
about  20  Ib.  per  ton.  If  the  floor  or  track  is  inclined  in  any  part 
of  the  travel,  the  grade  must  be  considered  in  addition  to  these 
figures.  The  weight  of  a  battery  truck  or  locomotive  varies 
considerably,  but  values  near  enough  for  estimating  the  loads  to 
be  carried  by  the  battery  are  the  following :  For  battery  trucks, 
up  to  10  tons  capacity,  1,000  Ib.  +  30  per  cent,  of  the  weight  of 
the  load  to  be  carried;  and  for  locomotives  with  drawbar  pull  up 
to  2,400  Ib.,  4,000  Ib.  +  4  times  the  drawbar  pull.  The  efficiency 
of  the  motor  is  about  80  per  cent,  at  any  ordinary  load.  This 
does  not  take  into  account  losses  due  to  speed  adjustment  by 
gear  reduction  or  by  controller  series  resistance;  but  it  does  per- 
mit speed  variation  with  a  ratio  of  about  3 : 4,  by  series  and  parallel 
connection  of  two  sections  of  the  series  field. 

Train  lighting  is  done  in  three  different  ways,  each  requiring 
the  use  of  storage  batteries: 

1.  The  straight  storage  system.     This  requires  a  sufficient  battery 
capacity  on  each  car  for  all  demands  for  light,  to  last  until  the 
car  is  brought  again  to  a  charging  station.     Usually  it  is  not  so 
satisfactory  as  one  of  the  other  systems.     It  is  proposed  to  stan- 
dardize on  a  nominal  voltage  of  60  (32  lead  cells)  for  this  system. 
A  number  of  roads  are  still  using  voltages  ranging  from  30  to  110, 
but  new  installations  should  be  at  60  volts  if  possible,  for  the 
sake  of  uniformity.     The  size  of  cell  most  commonly  used  has  a 
capacity  of  300  amp.-hr.     It  is  better  to  use  this  standard  size 
if  it  is  large  enough,  and  not  very  much  too  large. 

2.  The  head-end  system  has  a  generator,  in  the  baggage  car  or 
on  the  locomotive,  large  enough  to  light  the  whole  train.     The 
battery  need  not  be  so  large  in  this  case  as  in  the  straight  storage 
system,  because  it  is  only  required  to  furnish  lights  during  stops 
and  for  a  time  at  the  beginning  and  end  of  the  run,  for  cleaning 
and  other  work  about  the  cars.     Some  cars  require  light  for  longer 
times  before  and  after  the  run  than  others.     Thus,  mail  cars  are 


60  ELECTRICAL  EQUIPMENT 

required  to  have  sufficient  battery  capacity  to  light  them  for 
12  hr.  without  recharging  the  batteries;  diners  require  lights  for 
laying  in  supplies,  cleaning  and  other  operations;  Pullman  cars 
require  lights  for  cleaning,  and  for  occupancy  at  the  beginning  of 
the  run,  before  the  car  starts.  One  difficulty  of  the  head-end 
system  is  that  these  several  cars  cannot  be  charged  up  in  advance 
so  readily  as  if  each  were  handled  independently.  In  some  cases, 
but  not  all,  the  rule  can  be  followed  of  making  the  total  ampere- 
hour  capacity  of  the  battery  one-half  of  what  it  would  be  if  there 
were  no  generator  on  the  train.  This  system  has  another  dis- 
advantage where  the  train  is  broken  up  at  junction  points.  If 
some  cars  are  run  on  branch  lines  there  must  be  some  provision 
for  lighting  those  that  are  so  switched  off.  It  is  proposed  to 
standardize  the  battery  voltage  for  head-end  systems  on  60  volts. 
In  a  few  cases  110-volt  systems  are  in  use  at  present,  but  new 
systems  should  conform  to  the  standard.  Cells  of  300  amp.-hr. 
capacity  should  be  used  wherever  practicable,  but  others  may  be 
used  if  necessary. 

3.  The  axle-generator  system  has  both  a  generator  and  a  battery 
on  each  car,  and  is  therefore  the  most  flexible  of  all  systems.  The 
proposed  standard  for  this  system  is  30  volts.  At  present  dining 
cars  and  a  few  others  commonly  use  60  volts.  Cells  of  300  amp.- 
hr.  capacity  should  be  used  if  practicable.  The  same  provision 
must  be  made  as  before  indicated,  for  additional  time  of  lighting 
mail  cars,  diners  and  Pullman  sleepers. 


CHAPTER  IX 
ILLUMINATION1 
THE  ESSENTIALS 

It  requires  the  application  of  only  a  few  principles,  in  making 
the  necessary  computations  for  illumination.  We  shall  consider 
enough  of  these  principles  to  lay  out  the  equipment  for  good 
industrial  illumination. 

Illumination  intensity  refers  to  the  strength  of  light  on  the 
object  that  is  observed.  If  it  comes  from  a  single  lamp,  it  varies 
as  the  candlepower  and  inversely  as  the  square  of  the  distance 
of  the  lamp  from  the  surface  (G.  398,  399).  The  candlepower 
is  usually  different  in  different  directions,  and  the  value  used  in 
computing  should  be  found  for  the  required  direction,  from  a 
curve  that  shows  the  variation  of  candlepower  with  direction. 
In  Fig.  28  are  four  such  curves,  showing  the  candlepower  of  a 
100-watt  lamp  without  a  reflector,  and  with  three  different  kinds 
of  reflectors.  Illumination  intensity  is  expressed  in  foot-candles, 
and  a  surface  is  said  to  have  one  foot-candle  of  illumination  when 
the  light  is  from  a  one-candlepower  lamp,  one  foot  from  the 
surface,  if  the  beam  of  light  is  normal  to  the  surface.  The 
illumination  intensity  due  to  any  lamp  at  any  distance,  with 
the  light  striking  the  surface  at  any  angle,  is 

/  =  C  cos  6/D* 

where  C  is  the  candlepower  in  the  particular  direction,  D  the 
distance  in  feet  between  the  lamp  and  the  object,  and  6  the  angle 
between  the  light  beam  and  the  normal  to  the  surface.  If  the 

i  G.  Chapter  XLII. 

S.  Section  14;  also  see  list  of  references,  paragraph  250. 

A.  Theory,  pp.  764-771;  Interior  Illumination,  pp.  756-763;  Street 
Illumination,  pp.  772-778. 

Bulletins  of  Engineering  Department,  National  Lamp  Works  of  General 
Electric  Co.,  Cleveland,  O.  In  particular,  Bulletin  20,  Industrial  Lighting. 

Chapters  X  and  XI,  D.C.  and  A.C.  Lighting  Circuits. 

Chapter  VIII  and  references,  p.  51,  Train  and  Vehicle  Lighting. 

61 


62  ELECTRICAL  EQUIPMENT 

surface  is  lighted  by  several  sources,  the  total  intensity  is  the 
sum  of  the  intensities  from  the  several  sources. 

The  total  amount  of  light,  striking  any  surface,  is  the  product 
of  average  intensity  of  illumination  times  the  area.  If  the  in- 
tensity is  in  foot-candles,  and  the  area  in  square  feet,  the  total 
light,  or  the  light  flux,  is  expressed  in  lumens.  Consider  a  sphere, 
Fig.  26,  of  radius  D,  with  a  lamp  at  the  center  having  a  candle- 
power  C,  in  all  directions.  The  illumination  intensity  on  the 
inside  of  the  sphere  is  C/D2,  and  the  total  light  flux  in  lumens  on 
the  inner  surface  of  the  sphere  is  4irD2C/D*  or  4irC.  That  is, 
the  total  light  in  lumens  emanating  from  any  lamp  of  uniform 
candlepower  is  4?r  times  the  candlepower.  If 
the  candlepower  is  not  the  same  in  all  direc- 
tions, the  total  light  is  4?r  times  the  "mean 
spherical  candlepower."  Incandescent  lamps 
are  now  rated  in  mean  spherical  candlepower. 
Up  to  the  present  time  they  have  been  rated 
in  "horizontal  candlepower,"  which  is  the 
.  26. Lamp  of  candlepower  in  a  horizontal  direction  when 

C  candle-power   at    the  tip  points  straight  down.     This  is  about 
the     center     of     a     .«„„,.  ,      .     ,  „  ,        ,-,  . 

sphere  having  a  ra-    1-25  times  the  spherical  candlepower,  but  this 

dius  of  D  feet.  ratio  differs  in  different  lamps.     At  present, 

lamps  are  also  rated  in  lumens  or  in  watts 
or  both.  A  100-cp.  (spherical)  lamp  produces  4?r  X  100,  or 
1,257  lumens. 

The  effect  of  reflectors  is  to  change  the  direction  of  light,  and 
to  absorb  a  small  amount  of  it.  The  light  actually  reaching  the 
working  plane  is  increased  by  the  use  of  reflectors,  and  light  that 
would  be  annoying,  by  shining  in  the  eyes,  is  cut  off.  The  dis- 
tribution of  the  light  depends  on  the  kind  of  reflector  that  is 
used.  Reflectors  are  classified,  with  reference  to  distribution, 
as  extensive,  intensive,  and  focusing.  An  extensive  reflector 
throws  a  large  part  of  the  light  out  toward  the  horizontal,  and 
is  suitable  for  use  in  lighting  streets  and  large  areas.  An  intensive 
reflector  throws  the  light  on  an  area  that  is  less  extended;  it  is 
suitable  for  ordinary  industrial  lighting,  especially  for  purposes 
of  general  interior  illumination.  A  focusing  reflector  concentrates 
the  light  on  a  relatively  small  spot,  immediately  below  the  lamp. 
It  is  not  suitable  for  general  lighting  unless  the  lamps  are  placed 
close  together,  but  it  is  especially  good  for  spot  lighting,  where 
that  is  required. 


ILLUMINATION 


63 


The  shapes  of  several  of  these  reflectors  are  illustrated  in 
Fig.  27,  and  the  distribution  of  light  is  shown  in  Figs.  28  to  30. 


(a)  Intensi ve    ( 6)  Extensi ve 
Bowl  Shaped      Dome  Shaped 


(d)  Angle 


)  Concentric  Reflector  and. 
Prismatic  Glass  Refractor 


FIG.  27.— Typical  reflectors. 


Co),  (6)  and  (c)  are  made  of  steel,  covered  with  porcelain  or  other  reflecting  material. 
They  are  suitable  for  in-door  and  industrial  use.  Extensive  reflectors  are  usually  dome- 
shaped.  Intensive  reflectors  are  either  bowl-  or  dome-shaped;  the  dome-shaped  are  prefer- 
able on  accout  of  high  efficiency,  and  because  they  cast  softer  (less  sharp)  shadows.  The 
chief  disadvantage  of  dome  reflectors  is  that  they  do  not  conceal  the  lamp  filaments  from 
view.  See  Fig.  28  for  photometric  curves  of  these  reflectors,  (d)  is  suitable  for  certain 
cases  of  special  lighting.  (See  Fig.  29.)  (e)  is  a  combination  reflector  and  refractor.  It  is 
suitable  for  out-door  lighting.  (See  Fie.  30.) 


FIG.  28. — Curves  showing  the  distribution  of  light  from  a  clear  Mazda  lamp, 
without  a  reflector,  and  with  three  kinds  of  steel  reflectors. 

Radii  are  candle-power.     This  was  a  100-watt  lamp,  operating  at  9.1  lumens  per  watt,  or 
1.38  watts  per  spherical  candle-power. 

The  dotted  line  in  Fig.  28  illustrates  the  fact  that  the  candle- 
power  of  a  Mazda  lamp  is  relatively  high  in  a  horizontal  direc- 


64 


ELECTRICAL  EQUIPMENT 


FIG.  29. — Curve  of  distribution  of  light  from  a  100-watt  lamp  (9.1  lumens 
per  watt)  with  an  angle-type  reflector. 


FIG.  30. — Curve  of  distribution  from  an  85  spherical  candle-power  series 

Mazda  lamp. 

The  power  required  varies  somewhat,  depending  on  the  current  for  which  the  lamp  is  rated. 
It  is  about  0.8  watt  per  spherical  candle. 


ILLUMINATION 


65 


tion,  and  that  it  is  very  low  directly  downward.  The  exten- 
sive reflector  throws  a  stronger  light  than  the  intensive,  at  angles 
of  more  than  45°  from  the  vertical,  and  the  focusing  reflector 
throws  more  than  either  of  the  others  at  less  than  30°.  The 
first  impression  in  comparing  these  curves  would  be  that  the 
total  amount  of  light  is  several  times  as  great  with  the  focusing 
reflector  as,  with  the  others.  This  is  not  the  case,  because  the 
solid  angle  is  so  small  in  which  the  light  from  the  focusing  reflector 
is  very  strong. 

As  the  light  leaves  the  lamp  and  reflector,  a  large  part  of  it  is 
thrown  directly  on  the  working  plane.1  The  rest  falls  on  the 
walls,  ceiling  and  other  surfaces,  and  a  part  is  reflected  from 
there  to  the  working  plane.  The  utilization  efficiency  is  the  ratio 
of  the  light  flux  finally  reaching  the  working  plane  to  the  total 
light  produced.  Thus,  if  1,000  lumens  are  produced  by  the  lamp, 
and  the  average  illumination  intensity  is  3  foot-candles  on  an 
area  of  100  sq.  ft.,  the  utilization  efficiency  for  that  area  is  30 
per  cent.  The  efficiencies  in  Table  IX  are  given  by  the  National 
Lamp  Works  of  the  General  Electric  Co.;  they  show  that  in  a 
large  room  having  several  rows  of  units  in  each  direction,  two 
or  three  times  as  much  of  the  light  reaches  the  working  plane  as 
in  a  very  small  room  having  only  a  single  lamp. 

TABLE  IX. — UTILIZATION  EFFICIENCIES  OF  ILLUMINATION 


Installation  —  units  spaced  1.5  to 
1.6  times  height  above  work 

Reflector 

Enameled  steel 
dome,  per  cent. 

Enameled  steel  or 
pyro  glass  bowl, 
per  cent. 

1  unit  

28 
42 
48 
52 
56 
60 
63 
67 
71 

24 
36 
41 
44 
47 
49 
51 
54 
57 

1  row  of  5  units 

2  rows  of  2  units  

2  rows  of  3  units  .  .  . 

3  rows  of  3  units  

3  rows  of  4  units  

4  rows  of  4  units 

4  rows  of  8  units  

8  rows  of  8  units  . 

The  intensity  of  illumination  that  is  suitable  for  industrial 
purposes  depends  on  considerations  mentioned  later — particu- 

1  That  is,  the  horizontal  plane  at  the  average  height  of  the  work — usually 
30  to  40  in.  above  the  floor. 


66  ELECTRICAL  EQUIPMENT 

larly  on  glare.  It  is  sometimes  easier  to  see  in  a  room  that  has 
an  illumination  intensity  of  1  or  1.5  foot-candles,  produced  by 
indirect  lighting,  than  in  a  room  having  3  foot-candles  produced 
by  direct  lighting  in  which  the  lamp  filaments  are  in  the  range  of 
vision.  If  the  filaments  are  concealed,  and  the  lamps  are  placed 
as  much  as  possible  out  of  the  field  of  vision,  the  intensities  given 
in  Table  X  should  be  sufficient,  for  ordinary  cases;  but  special 
conditions  may  call  for  either  higher  or  lower  values. 

TABLE  X. — ILLUMINATION  INTENSITIES 


Purpose  of  illumination 

Average  values  for 
well-placed  lamps 
with  suitable  re- 
flectors, foot-candles 

Desk  work  

4-6 

Fine  machine  work 

5-10 

Rough  machine  work  

3-5 

Storage  

1 

Passageways 

1-2 

The  effect  of  dust  and  aging  of  lamps  must  be  taken  into  account 
in  providing  for  the  illumination  of  a  room  or  other  space.  The 
values  of  illumination  intensity  given  in  Table  X  refer  to  condi- 
tions that  should  be  found  in  service,  after  dust  has  accumulated 
on  the  lamps  and  reflectors,  and  the  lamps  have  become  somewhat 
dimmed  with  age.  The  effect  of  dust  depends  on  the  frequency  of 
cleaning  the  lamps,  the  location  of  lamps,  general  conditions  as 
to  dust,  and  the  kind  of  reflectors  used.  An  average  dimming  on 
account  of  dust,  with  the  kind  of  reflectors  commonly  used  in 
industrial  plants,  is  1  per  cent,  per  week  for  the  first  2  months, 
and  a  further  dimming  of  0.5  per  cent,  per  week  for  the  next  4 
months.1  The  aging  of  a  Mazda  (tungsten)  lamp,  during  a  period 
of  1,000  hr.  of  use  which  is  considered  the  normal  length  of  life 
of  the  lamp,  produces  a  decrease  in  lumens  to  87  per  cent.2  of 
the  initial  value,  at  the  end  of  the  life  of  the  lamp.  The  average 
during  the  life  of  the  lamp  is  94.5  per  cent.2  of  the  initial  value. 
If  we  are  willing  to  accept  average  illumination  during  the 

1  These  figures  are  for  ordinary  conditions.     Under  very  bad  conditions 
there  may  be  a  dimming  of  50  per  cent,  in  1  month. 

2  These  figures  show  the  decrease  in  useful,  as  well  as  in  total  light,  if 
dome  reflectors  are  used;  but  with  bowl  reflectors,  the  useful  light  drops  in 
1,000  hr.  to  82  per  cent,  of  the  initial  value,  and  the  average  during  the  life 
of  the  lamp  is  92  per  cent. 


ILLUMINATION  67 

1,000  hr.,  we  shall  allow  for  the  average  effect  of  dust  and  aging; 
but  if  the  minimum  illumination  intensity  is  not  to  fall  below  the 
specified  value,  it  is  necessary  to  allow  for  the  accumulation  of 
dust  during  the  full  period  between  cleanings,  and  the  effect  of 
aging  in  the  full  1,000  hr. 

If  the  lamps  are  not  spaced  too  far  apart,  and  suitable  re- 
flectors are  used,  the  illumination  is  practically  uniform  over  the 
entire  area.  The  distances  given  in  Table  XI  should  not  be 
exceeded,  if  uniform  illumination  is  required. 

TABLE  XI. — MAXIMUM  SPACING  DISTANCES  FOR  UNIFORM  ILLUMINATION 
(H  is  the  height  of  the  lamps  above  the  working  plane) 

Kind  of  reflector  Distance  between  rows 

Extensive 2.  OH 

Intensive ' 1 . 25H 

Focusing Q.75H 

Glare. — The  purpose  of  light  is  to  make  objects  visible,  and 
we  should  consider  not  only  what  intensity  of  light  is  produced, 
but  also  whether  anything  reduces  the  usefulness  of  the  light. 
If  the  eye  is  accustomed  to  a  very  bright  light,  it  is  not  in  condi- 
tion to  see  very  well  on  a  moderately  lighted  surface.  Any 
such  interference  with  clear  vision  is  called  glare.  There  are 
several  cases  to  consider:  (1)  Bright  spot-lighting  contracts  the 
pupil  of  the  eye,  and  makes  it  difficult  to  see  objects  in  the  vicin- 
ity, even  if  they  are  fairly  well  lighted.  (2)  A  reflecting  surface 
sometimes  produces  glare,  if  the  light  strikes  it  at  such  an  angle 
as  to  be  reflected  to  the  eye.  (3)  The  intensely  bright  filament  of 
a  tungsten  lamp,  or  the  arc  of  an  arc  lamp,  if  not  covered,  pro- 
duces glare  when  it  is  seen  in  looking  less  than  about  20°  above 
the  horizontal.  (4)  A  flickering  light  is  similar  to  the  other  cases, 
and  in  addition  the  continuous  changing  tires  the  eye. 

Color  of  light  has  a  considerable  effect  on  its  usefulness;  for 
most  industrial  purposes  the  colors  of  tungsten  and  arc  lighting 
are  satisfactory.  If  the  work  requires  careful  color  observations 
a  test  should  be  made  before  the  installation  is  completed,  to 
ascertain  what  type  of  lamp  is  most  effective,  and  causes  the 
least  eye-strain. 

Shadows. — If  an  object  is  so  perfectly  lighted  that  there  are 
no  shadows,  the  details  of  the  object  are  not  so  plainly  visible 
as  if  there  are  moderate  shadows,  showing  by  contrast  where  the 
depressions  and  projections  are.  On  the  other  hand,  if  shadows 


68  ELECTRICAL  EQUIPMENT 

are  too  intense,  the  part  in  the  shadow  is  entirely  invisible.  The 
best  effect  is  obtained  if  the  light  comes  from  at  least  two  or  three 
directions.  For  drafting,  and  similar  work  on  a  plane  surface, 
the  less  shadows  there  are,  the  better  is  the  effect. 

THREE  KINDS  OF  ILLUMINATION 

We  have  found  that  sufficient  intensity,  avoidance  of  glare, 
and  moderate  shadows  are  essential  to  good  lighting.  Each  of 
these  is  obtained  to  a  greater  or  less  degree  by  each  kind  of 
illumination : 

Direct  lighting  has  the  advantage  over  indirect,  that  the 
light  is  used  more  efficiently.  It  has  the  disadvantage  that  the 
source  of  light  is  visible,  and  may  produce  glare.  For  the  best 
results  the  lamps  must  be  out  of  the  field  of  vision,  and  close 
enough  together  so  that  the  working  plane  is  uniformly  lighted 
and  so  that  several  lights  from  different  directions  show  the  form 
of  every  object  by  the  shadows. 

Indirect  lighting  is  more  expensive  on  account  of  inefficiency. 
The  surfaces  lighted  should  be  large  enough  to  reflect  sufficient 
light  without  producing  glare.  For  even  moderate  efficiency, 
the  lighted  surfaces  should  be  of  a  very  light  color. 

Semi-indirect  lighting  is  produced  by  lamps,  at  least  part 
of  which  are  used  for  direct  lighting,  whereas  some  or  all  throw 
their  light  also  on  walls,  ceilings  or  other  surfaces,  for  indirect 
lighting.  If  the  lamps  are  screened  from  the  eye  by  an  adequate 
diffusing  medium,  this  kind  of  light  may  be  very  nearly  as  soft 
and  free  from  glare,  as  indirect  lighting,  and  it  is  less  expensive. 

COMPUTATIONS 

There  are  two  methods  of  finding  the  number,  size  and  arrange- 
ment of  lamps,  to  produce  the  required  illumination.  The 
point-by-point  method  is  very  tedious,  and  is  as  follows:  a  trial 
layout  is  made,  of  lamps  and  reflectors  such  as  would  be  expected 
to  give  the  necessary  illumination  intensity  in  all  parts  of  the 
room  or  space.  The  intensity  at  a  certain  point  is  then  computed 
by  using  the  formula  on  page  61;  it  is  the  sum  of  all  the  inten- 
sities at  that  point  from  all  the  sources  in  the  vicinity.  There 
may  be  a  dozen  lamps  whose  effect  at  that  point  is  to  be  computed. 
Then  all  these  computations  must  be  repeated  at  a  large 


ILLUMINATION  69 

number  of  points,  so  that  the  illumination  in  every  part  of  the 
room  is  known.  If  a  part  or  all  the  illumination  is  unsatisfac- 
tory, it  must  be  changed,  and  new  computations  made. 

A  much  simpler,  and  quite  satisfactory  method  is  that  of  ob- 
taining the  average  illumination  intensity.  Reflectors  have  been 
so  well  developed  that  where  proper  spacing  is  not  exceeded  the 
illumination  is  practically  uniform.  If  we  assume  a  convenient 
height  of  lamps,  the  table  of  spacing  distances  on  page  67  gives 
us  the  maximum  allowable  spacing  between  rows,  and  from  that 
we  know  the  area  to  be  lighted  by  each  lamp.  The  product  of 
area  and  illumination  intensity  gives  lumens  per  lamp.  Or,  if  we 
assume  a  convenient  size  of  lamp,  we  know  the  spacing  and  the 
minimum  allowable  height  of  lamps.  The  problem  is  one  of 
finding  consistent  values  of  height,  spacing  and  lumens  of  each 
lamp.  If  a  satisfactory  solution  is  not  obtained  directly,  it  may 
be  desirable  (1)  to  specify  a  spacing  that  is  less  than  the  maximum 
allowable,  (2)  to  provide  an  illumination  of  higher  intensity,  or 
possibly  slightly  lower  than  was  originally  required,  or  (3)  to 
provide  general  illumination  that  is  considerably  too  weak,  but 
is  supplemented  in  certain  localities  by  any  method  that  is  indi- 
cated below  for  special  lighting.  In  any  case  computations  are 
made  for  average  illumination  on  the  working  plane,  taking  into 
account  the  effect  of  dust  and  aging  of  lamps  and  the  utilization 
efficiency. 

In  laying  out  the  positions  of  the  lamps,  the  scheme  can  some- 
times be  modified  to  adapt  it  to  the  shape  of  the  room  and  the  lo- 
cation of  machines,  preventing  dark  corners  and  edges  of  the 
room,  if  good  light  is  required  in  these  places.  The  distance  from 
the  wall  to  the  first  row  of  lamps  should  not  be  more  than  one-half 
the  distance  between  rows,  unless  good  illumination  is  unneces- 
sary near  the  wall.  Even  if  the  distance  between  the  first  row 
and  the  wall  is  as  little  as  one-half  the  spacing  between  rows,  the 
illumination  is  appreciably  less  near  the  wall,  unless  the  wall  is 
of  a  very  light  color,  and  serves  as  a  good  reflector. 

The  provision  thus  far  is  for  general  illumination.  If  this  is 
not  sufficient  for  all  parts  of  the  room,  special  lighting  may  be 
introduced  to  increase  the  illumination  in  certain  sections  (1)  by 
increasing  the  size  of  lamps;  (2)  by  reducing  the  spacing  between 
lamps,  (3)  by  using  focusing  reflectors,  or  (4)  by  providing  addi- 
tional hand  or  stationary  lamps.  The  use  of  stationary  lamps 
probably  gives  the  most  satisfactory  lighting  for  a  small  amount 


70  ELECTRICAL  EQUIPMENT 

of  power  used.1  In  a  shop  or  factory,  the  lamp  should  then  be 
placed  where  it  lights  the  work  to  the  best  advantage.  In 
general,  a  good  rule  is  to  place  the  lamp  so  that  when  the  operator 
is  at  work,  it  is  just  to  the  right  and  in  front  of  his  right  shoulder, 
and  just  above  his  head.  The  best  height  of  the  lamp  is  from 
4  to  7  ft.  above  the  floor,  depending  on  whether  the  operator  is 
seated  or  standing.  It  must  be  placed  where  no  shadow  is  cast 
on  the  work. 

1  Any  such  localized  lighting  is  rarely  necessary.     It  nearly  always  pro- 
duces glare  in  some  form,  and  should  be  avoided  if  possible. 


CHAPTER  X 
D.C.  TRANSMISSION  AND  DISTRIBUTION  SYSTEMS1 

This  chapter  applies  very  largely  to  A.C.  as  well  as  to  D.C. 
systems ;  some  further  points  are  brought  out  in  the  next  chapter, 
with  reference  to  A.C.  circuits.  It  is  assumed  that  the  kind  of 
system  has  been  chosen  in  accordance  with  Chapter  III ;  we  now 
proceed  to  find  the  size  of  wire,  which  must  be  large  enough  so 
that  the  current  does  not  produce  (1)  excessive  voltage  drop, 
(2)  excessive  power  loss  in  the  line,  nor  (3)  excessive  heating  of 
the  conductor.  It  must  also  be  large  enough  for  mechanical 
strength,  but  not  so  large  that  the  investment  is  unnecessarily 
large. 

VOLTAGE  DROP 

Motor  Circuits. — When  the  voltage  of  a  system  drops,  the 
motor  current  must  increase,  if  the  motor  is  to  do  the  same  work 
as  at  full  voltage;  also,  the  operating  characteristics  of  some 
motors  are  impaired.  A  drop  of  5  per  cent,  is  satisfactory  in 
motor  circuits  under  practically  all  conditions,  and  usually  10 
per  cent,  is  not  excessive.  Still  greater  voltage  drop  may  be 
necessary  in  extreme  cases,  but  should  not  be  allowed  without 
careful  consideration  of  the  extra  cost  and  the  advantage  of 
keeping  it  within  10  per  cent.2 

Lighting  Circuits. — The  voltage  drop  should  be  even  smaller 
for  lighting  circuits  than  for  motors,  because  every  1  per  cent, 
decrease  in  voltage  causes  from  3  to  4  per  cent,  decrease  in  the 

1  G.  Chapter  XLL 

S.  Section  11;  High-tension  long-distance  transmission. 

Section  12;  Distribution  systems  and  short  transmission  lines. 

Section  13;  Interior  wiring  and  local  distribution. 
A.  pp.  352-376,  1657-1707. 

2  Overcompounded  generators,  voltage  regulators,  or  boosters  can  be 
employed  to  maintain  a  steady  voltage,  but  none  of  these  will  keep  the  vol- 
tage constant  on  all  parts  of  the  system  without  a  large  investment  in  equip- 
ment.    If  possible,  the  line  drop,  independent  of  automatic  regulating  de- 
vices, should  not  be  excessive. 

71 


72  ELECTRICAL  EQUIPMENT 

amount  of  light  produced.  For  example,  if  the  voltage  drops 
5  per  cent.,  the  candlepower  of  a  tungsten  lamp  decreases  about 
16  per  cent.  For  this  reason,  the  voltage  drop  on  a  lighting 
circuit  should  not  ordinarily  exceed  3  per  cent.,  and  it  is  better 
not  to  exceed  2  per  cent. 

There  are  three  cases  of  voltage  drop  to  be  considered,  for  D.C. 
motor  and  lighting  circuits:  (1)  The  two-wire  system,  in  which 
all  the  current  flowing  out  on  one  wire  necessarily  returns  on  the 
other;  (2)  the  ground-  or  rail-return  system,  in  which  the  cur- 
rent flowing  out  on  the  copper  wire  returns  through  the  ground, 
or  over  rails,  or  both;  and  (3)  the  three- wire  and  other  multiple- 
voltage  systems,  in  which  at  least  one  additional  conductor  at  an 
intermediate  voltage  is  provided  for  lighting,  or  for  motor-speed 
adjustment. 

Two-wire  System. — If  a  feeder  is  very  long  and  has  branches 
taking  considerable  parts  of  the  total  current,  the.  conductor  need 
not  be  so  large  at  the  end  as  at  the  beginning  of  the  feeder.1 
Ordinarily,  however,  it  does  not  pay  to  make  the  joints  and  to 
change  sizes;  a  size  is  selected  which  is  large  enough  for  safety 
and  economy,  and  which  distributes  the  current  to  its  various 
destinations  without  producing  a  drop  at  any  destination,  ex- 
ceeding the  allowable  maximum.  The  total  RI  drop  in  a  D.C. 
line  is  computed  by  adding  together  the  RI  drops  in  the  various 
parts  that  are  in  series. 

Example. — A  feeder  of  500,000  circ.  mil  cable  furnishes  power  to  three 
motors,  at  distances  along  the  feeder  of  50,  100  and  200  ft.  from  the  busbars. 
The  motors  take,  respectively,  100,  200  and  250  amp.  It  is  required  to  find 
the  voltage  drop.  From  Table  XII,  p.  80,  the  resistances  of  the  three 
lengths  are  respectively  0.00216,  0.00216,  and  0.00432  ohms.  The  total 
drop  is  550  X  0.00216  +  450  X  0.00216  +  250  X  0.00432  or  3.24  volts. 
It  is  sometimes  simpler  to  compute  the  drop  due  to  the  individual  currents 
by  multiplying  each  by  the  total  resistance  through  which  it  flows.  The 
total  drop,  computed  by  that  method,  is  0.00216  X  100  +  0.00432  X  200  + 
0.00864  X  250,  which  agrees  with  the  other  computations. 

The  voltage  drop  may  be  computed  without  the  use-  of  the 
table,  as  equal  to  kLI/A,  where  k  is  the  resistance  of  a  circular- 

1  In  such  a  case  it  can  be  shown  that  the  most  economical  distribution 
of  copper  for  minimum  line  drop  is  obtained  if  the  sectional  area  of  each 
length  of  conductor  is  proportional  to  the  square  root  of  the  current  in  that 
length.  Thus,  if  the  first  100  ft.  carries  nine  times  as  much  current  as  the 
second,  and  we  consider  minimum  line  drop  and  nothing  else,  the  sectional 
area  of  the  first  100  ft.  should  be  three  times  as  great  as  that  of  the  second. 


D.C.  TRANSMISSION  AND  DISTRIBUTION        73 

mil-foot  (10.6  ohms,  for  annealed  copper  at  25°C.),  L  is  the 
length  of  conductor  in  feet,  I  the  current  in  amperes,  and  A  the 
area  in  circular  mils.  It  is  simpler  to  use  the  table  than  to  make 
this  computation  to  find  the  voltage  drop,  but  if  the  voltage  drop 
is  given,  and  the  size  of  conductor  is  to  be  found,  it  may  be  simpler 
to  use  this  formula  than  to  try  the  various  sizes  of  wire  until  the 
one  is  found  that  gives  the  right  voltage  drop.1  Where  it  is 
specified  that  the  voltage  drop  shall  not  exceed  a  certain  maxi- 
mum, of  course  the  full-load  value  of  the  current  is  to  be  sub- 
stituted in  the  expression  for  voltage  drop,  even  though  the 
average  current  is  much  smaller.  However,  this  maximum  drop 
does  not  necessarily  refer  to  the  period  of  heavy  currents  for 
motor  starting,  lasting  for  a  fraction  of  a  minute;  for  even  if 
such  currents  cause  an  excessive  drop  for  a  very  short  time,  they 
need  not  interfere  with  satisfactory  operation  of  other  machines. 

Example. — If  the  total  full-load  current  taken  by  all  the  motors  on  a 
feeder  is  500  amp.,  the  motors  are  200  ft.  from  the  power-station  buses,  and 
the  maximum  allowable  drop  is  15  volts,  the  required  area  of  the  wire  is 
10.6  X  2  X  200  X  500/15  or  141,000  circ.  mils.  From  the  table  we  find  that 
No.  000  wire  is  the  size  to  use. 

Ground  or  Rail  Return. — In  a  circuit  having  a  rail  or  other 
return  path,  if  the  drop  in  the  return  circuit  is  appreciable,  the 
resistance  of  the  return  circuit,  and  the  current  (if  it  is  different 
from  that  of  the  wire)  must  be  determined.  The  total  drop  is, 
then,  the  resistance  of  only  one  wire  times  its  current,  plus  the 
resistance  of  the  return  circuit  (if  appreciable)  times  its  current. 
If  the  return  circuit  is  through  a  rail,  the  drop  in  the  rail  is  usually 
small;  the  resistance  of  two  60-lb.  rails  in  parallel  is  about  0.0083 
ohm  per  1,000  ft.  The  resistance  of  other  weights  of  rail  is 
very  nearly  inversely  as  the  weight. 

1  For  rapid  calculation  of  wire  resistances  without  reference  to  tables, 
the  following  rules  are  convenient  to  memorize.  At  20°C.  (68°F.),  for  ordi- 
nary commercial  copper  wire  of  sizes  from  No.  0000  to  10,  A.W.G.,  they 
are  correct  within  2  per  cent.  The  errors  are  slightly  larger  for  smaller 
wires. 

Rule  1. — The  resistance  of  No.  10  wire  is  1  ohm  per  1,000  ft.;  adding  3 
to  the  number  of  any  wire  doubles  the  resistance;  and  subtracting  3  from  the 
number  halves  the  resistance.  That  is,  changing  the  number  by  1  multiplies 
or  divides  the  resistance  by  v^2  or  1.26. 

Rule  2.  Adding  10  to  the  number  of  any  wire  multiplies  the  resistance 
by  10,  and  subtracting  10  divides  it  by  10. 


74  ELECTRICAL  EQUIPMENT 

Thus  the  drop  in  the  rail  return  1  mile  long,  consisting  of  two  40-lb.  rails 
in  parallel,  when  carrying  150  amp.,  is  150  X  0.0083  X  5.28X40X60  or 
4.4  volts.  If  only  one  rail  is  used  as  the  return  circuit,  of  course  the  drop 
is  twice  as  great. 

Multiple-voltage  Systems. — A  direct-current  three-wire  circuit 
should  be  treated  the  same  as  a  two-wire  circuit,  if  the  circuit  is 
balanced.  The  current  in  one  line  and  the  voltage  between 
outside  lines  should  be  used  in  computing  per  cent,  voltage  drop. 

Example. — If  100  lamps  each  taking  1  amp.  at  110  volts  are  balanced  on  a 
110-  and  220-volt  three-wire  circuit,  there  are  50  lamps  on  each  side,  and 
the  per  cent,  line  drop  is  the  same  as  for  50  amp.  on  a  220-volt  circuit. 

If  a  three-wire  circuit  is  unbalanced,  the  voltage  on  either 
side  of  the  system  may  be  .either  too  high  or  too  low.  (See  the 
three-wire  feeder  in  Fig.  14.)  If  there  is  a  larger  current  on 
the  positive  than  on  the  negative  side,  a  part  of  the  current 
returns  through  the  neutral.  Designating  the  positive,  neutral 
and  negative  currents  by  I+}  In,  and  /_,  and  the  resistances  of 
the  outside  and  neutral  wires  by  R0  and  Rn,  the  drop  in  the  posi- 
tive line  is  R0I+  +  Rnln-  In  is  usually  a  small  fraction  of  /+, 
but  Rn  may  be  larger  than  R0.  A  numerical  example  will 
illustrate : 

Assume  that  on  a  110-  and  220-volt  system  the  maximum  current  that  will 
flow  in  an  outside  line  is  50  amp.  and  at  least  80  per  cent,  of  this  current  is 
balanced  by  a  current  returning  in  the  other  outside  line.  If  the  resistance 
of  the  outside  line  is  0.04  ohm,  and  the  neutral  0.08  ohm,  the  maximum  vol- 
tage drop  with  balanced  load  is  0.04  X  50,  which  amounts  to  2  volts  on  each 
side,  or  4  volts  on  both  sides.  This  is  a  drop  of  1.8  per  cent.  The  maxi- 
mum drop  with  unbalanced  load  is  0.04  X  50  +  0.08  X  10  or  2.8  volts  on 
one  side.  This  is  a  drop  of  2.5  per  cent.  Note  that  if  /»  is  reversed,  the 
second  term  of  the  voltage  drop  is  negative. 

ECONOMICAL  SIZE  OF  WIRE 

Even  if  a  large  conductor  is  not  required  for  any  other  reason, 
it  may  be  required  for  economy.  Evidently  a  very  small  con- 
ductor is  not  economical,  because  the  annual  power  lost  is  pro- 
portional to  the  resistance,  or  inversely  proportional  to  the 
sectional  area  of  the  wire.  On  the  other  hand,  a  very  large  con- 
ductor costs  so  much  that  there  is  an  excessive  annual  outlay 
for  interest  and  other  fixed  charges — that  is,  for  charges  that 
exist  whether  the  conductors  are  carrying  current  or  not.  There 
is  an  intermediate  size  of  wire  that  is  most  economical,  whose 


D.C.  TRANSMISSION  AND  DISTRIBUTION        75 

exact  size  would  be  dependent  on  the  cost  per  kilowatt-hour  for 
energy,  and  on  the  necessary  allowance  for  fixed  charges,  which 
include  interest,  taxes,  insurance  and  depreciation. 

Some  of  the  items  of  cost  in  installing  a  transmission  or  dis- 
tribution system  are  the  same  for  any  ordinary  size  of  wire. 
Other  items  are  about  proportional  to  the  weight,  and  therefore 
to  the  sectional  area  of  the  wire.  Since  the  fixed  charges  are  a 
certain  per  cent,  of  the  first  cost,  some  fixed  charges  are  constant, 
whereas  others  are  proportional  to  area  of  conductor.  Thus 
the  total  annual  outlay  on  account  of  the  line  includes  the  fixed 
charges,  in  two  parts,  and  the  cost  of  energy.  It  may  be  ex- 
pressed as 

C  =  K,  +  K2A  +  KS/A 

where  A  is  the  area,  KI  the  invariable  fixed  charges,  KZA  the 
annual  fixed  charges  proportional  to  the  area,  and  K3/A  the  cost 
of  energy  lost  on  the  line  per  year.  Differentiating  the  annual 
outlay,  with  respect  to  area,  and  setting  the  first  derivative  equal 
to  zero,  to  find  the  area  for  minimum  cost,  we  have 

dC/dA  =  K2-  KS/A2  =  0 

from  which  we  have  K$A  =  KZ/A. 

That  is,  the  annual  fixed  charges  that  are  proportional  to  the  area 

should  equal  the  cost  of  energy  lost  on  the  line  per  year. 

The  cost  of  energy  lost  is  RIHCe/ 1,000,  where  t  is  the  number 
of  hours  per  year  that  the  current  flows,  and  Ce  is  the  cost  of  en- 
ergy in  cents  per  kilowatt-hour.  If  the  line  is  of  annealed  cop- 
per, the  resistance  is  10.6L/A  where  L  is  the  length  in  feet 
and  A  the  area  in  circular  mils;  and  the  cost  of  energy  lost  is 
10.6LIW./1,OOQA. 

The  fixed  charges  are  3.03  X  10~6  LACJF,  where  3.03  X  10~6  is 
the  weight  of  a  circular-mil-foot  of  copper,  Cc  the  cost  of  copper, 
installed,  in  cents  per  pound,  and  F  the  fraction  to  be  allowed 
annually  for  fixed  charges.  Equating  the  fixed  charges  to  the 
cost  of  energy,  and  solving  for  area,  if  t  is  365  X  24  hr., 

(2) 

When,  as  is  usually  the  case,  the  value  of  A  is  not  a  commercial 
size  of  wire,  the  nearest  size  should  be  selected — not  necessarily 

1  See  S.  13:  75,  76  for  a  similar  statement,  based  as  this  is  on  Kelvin's  law. 


76  ELECTRICAL  EQUIPMENT 

the  next  larger  size.  If  the  time  of  operation  per  year  is  not 
365  days  of  24  hr.,  the  area  is  proportional  to  the  square  root  of 
the  time. 

Thus,  if  a  line  is  in  service  only  8  hr.  per  day,  300  days  per  year,  expression 
(2)  becomes 

A  =  5,500  X  V8  X  300/(24  X  365)  X  VCe/(CtF). 

The  cost  of  energy  per  kilowatt-hour,  Ce,  is  usually  between 
1  and  10  cts.  if  purchased  from  a  power  company.  If  not  pur- 
chased, but  generated  in  a  plant  of  2,000-kw.  capacity  or  more, 
it  should  ordinarily  cost  from  0.5  to  1  ct.  per  kw.-hr.,  depending 
largely  on  the  size  of  the  plant  and  the  cost  of  coal. 

The  cost  of  copper  per  pound,  installed,  Cc,  including  insulated 
wire,  supplies  and  the  labor  of  wiring,  depends  on  the  prevailing 
base  on  which  wire  costs  are  computed,  size  of  wire,  the  kind  of 
wire  insulation,  discounts  obtainable,  cost  of  labor,  and  kind  of 
wiring  system — that  is,  whether  an  out-of-doors  pole  line,  an 
in-doors  conduit  system,  or  some  other  kind  of  installation. 
This  total  cost  may  be  as  low  as  25  cts.  per  Ib.  of  copper  installed, 
or  as  high  as  75  cts.,  for  such  sizes  as  are  used  for  power  purposes. 
To  find  this  total  cost,  proceed  as  follows : 

1.  Knowing  from  market  quotations  the  base  on  which  the  re- 
quired wire  is  sold,  find  from  the  price  list  in  Table  XII  the  list 
price  of  the  wire  per  1,000  ft.,  and  take  off  whatever  discount  is 
allowed. 

2.  Add  to  this  the  cost  per  1,000  ft.  for  conduits  or  other  sup- 
plies, and  labor. 

3.  Divide  by  the  weight  of  bare  wire,  in  pounds  per  1,000  ft. 

It  is  required  to  find  the  total  cost  of  a  pole  line,  not  including  poles,  per 
pound  of  copper.  Market  quotations  for  the  required  wire  are  on  the  15-ct. 
base,  and  a  discount  of  45  per  cent,  is  obtainable.  The  line  is  to  be  of  No.  4 
stranded  conductor. 

List  price  of  wire  per  1,000  ft.  is       $122.00 

Taking  off  the  discount,  122  X  0.55  is     67 . 10 
Labor  and  supplies  cost,  per  1000  ft.      25.00 

Total  cost  per  1000  ft.  $92 . 10 

Dividing  by  the  weight  in  pounds,  the  - 
cost  per  pound  of  copper  is  0.71 

The  rate  of  interest  is  usually  about  5  or  6  per  cent.,  depending 
on  financial  condition. 


D.C.  TRANSMISSION  AND  DISTRIBUTION        77 

Taxes,  of  course,  depend  on  the  locality.  An  allowance  of 
1.5  per  cent,  is  reasonable. 

Fire  insurance  is  placed  on  buildings  and  their  contents,  and 
other  equipment  that  may  be  destroyed  by  fire;  but  it  is  not 
customary  to  insure  transmission  and  distribution  lines  that  are 
outside  of  buildings.  Insurance  on  power  stations  and  other 
equipment  ranges  from  practically  zero  to  1.5  per  cent.  In  a 
well-constructed  building  it  is  not  far  from  0.5  per  cent. 

Depreciation  is  an  allowance  for  a  decrease  in  value,  due  to 
ordinary  wear  and  tear,  effect  of  the  weather,  and  being  displaced 
by  equipment  better  adapted  to  the  requirements.  (Scrap  value 
of  old  equipment  reduces  the  necessary  allowance  for  deprecia- 
tion.) It  is  not  customary  to  charge  maintenance  and  repairs 
to  depreciation,  but  these  charges  should  be  included  here  if  they 
are  not  elsewhere.  Allowance  for  depreciation  ranges  in  most 
cases  from  5  to  10  per  cent.,  but  in  a  few  cases  it  goes  much  higher 
or  lower.  For  ordinary  wires  and  wiring  equipment  (not  includ- 
ing trolley  wires)  it  is  about  7  per  cent.  This  brings  the  total 
fixed  charges  for  an  out-of-doors  circuit  to  about  14  per  cent. 
(It  usually  ranges  between  12  and  16  per  cent.)  This  percent- 
age allowed  for  fixed  charges  is  to  be  substituted  for  F  in  equa- 
tion (2).  It  is  to  be  written  as  a  decimal  fraction,  e.g.,  0.14, 
not  as  a  whole  number,  as  14  per  cent. 

A  good  check  on  the  results  of  equation  (2)  may  be  made  by 
comparing  the  total  annual  cost  for  the  chosen  size  with  that  for 
the  next  sizes  above  and  below. 

Example. — Let  us  apply  the  formula  and  then  check  it,  to  find  the  most 
economical  size  of  triple-braid  weatherproof  wire  to  carry  300  amp.,  9 
hr.  per  day,  295  days  per  year,  over  an  out-of-door  pole  line,  if  this  wire  is 
sold  on  the  20  ct.  base,  there  is  a  discount  of  53  per  cent.,  fixed  charges  are  14 
per  cent,  and  energy  costs  1  ct.  per  kw.-hr. 

Usually  results  come  out  at  about  1,000  to  2,000  circ.  mils  per  amp.,  so 
we  shall  look  for  a  conductor  of  300,000  to  600,000  circ.  mils.  Let  us  work 
the  problem,  using  the  data  for  500,000  circ.  mils.  From  Table  XII  and 
Note  9  of  that  table,  we  find  that: 

Cost  of  the  conductor  per  1,000  ft.  is  $604 

Cost  of  labor  and  supplies  per  1,000  ft.  is          $28 


Total  cost  per  1,.000  ft.  is  $632 


Weight  in  pounds  per  1,000  ft.  is  1,540 

Cost  per  pound  is  $0.41 


78  ELECTRICAL  EQUIPMENT 

Substituting  this  value  for  Cc,  in  equation  (2),  we  have 


VI  Q  V  2Q^ 

40<"al4X24  X  365  =  379'000  circ'  mils' 

The  best  commercial  size  is  400,000  circ.  mils. 

This  solution  was  obtained,  by  finding  the  value  of  Cc  for  a  500,000-circ. 
mil  conductor.  This  would  be  so  nearly  the  same  for  400,000  circ.  mils  that 
usually  no  further  computations  are  necessary.  As  an  extra  precaution,  the 
solution  can  be  repeated,  after  finding  Cc  for  400,000  circ.  mils. 

Checking  the  foregoing  by  comparing  costs  for  400,000  circ.  mils  with  those 
for  350,000  and  450,000,  we  find: 


Area  in  circular  mils 

350  000 

400  000 

450  000 

Cost  of  insulated  wire 

$442  00 

$496  00 

$549  00 

Labor  and  supplies  

23.80 

25.50 

27.00 

Total  first  cost 

$465  80 

$521  50 

$576  00 

14  per  cent,  fixed  charges 

$65  20 

$73  10 

$80  60 

Cost  of  energy  lost  at  21°C  

72.40 

63.40 

56.30 

Total  annual  outlay  

$137.60 

$136.50 

$136.90 

These  results  agree  with  those  obtained  by  the  formula,  in  indicating 
that  400,000  circ.  mils  is  the  most  economical  size.  There  is  a  small 
theoretical  error  in  using  the  formula,  because  in  deriving  it  we  assumed 
that  the  cost  of  copper  per  pound  is  constant,  whereas  it  is  slightly  less  for 
large  sizes  than  for  small  sizes  of  wire.  For  this  reason,  results  obtained 
by  the  formula  are  usually  about  3  per  cent,  too  low. 

Variable  Current. — If  the  current  is  not  steady,  but  has  values 
Iit  It,  73,  .  .  .  respectively,  for  h,  tz,  h,  .  .  .  hr.  per  year, 
the  value  of  /  to  use  in  equation  (2)  is  equal  to 


\/A  +  1 V*2  +  I*%  +  •    -    .  /24  X  365. 

Center  of  Distribution  for  Branched  Circuits. — If  the  circuit 
branches  near  the  outer  end,  so  that  the  current  in  the  main 
feeder  is  smaller  at  the  end  than  at  the  beginning,  it  is  allowable 
to  make  computations  as  if  the  feeder  carried  all  the  current  to 
a  center  of  distribution,  whose  distance  from  the  power  buses 
is  less  than  that  of  the  farthest  load,  but  more  than  that  of  the 
nearest  load.  A  large  error  may  be  introduced,  however,  if  the 
current  branches  at  a  point  near  the  power  station. 


D.C.  TRANSMISSION  AND  DISTRIBUTION        79 

SAFE  SIZE  OF  WIRE 

The  wire  must  be  safeguarded  against  mechanical  strain  and 
electrical  heating.  The  National  Electrical  Code  lists  the  carry- 
ing capacities  allowed  by  the  National  Board  of  Fire  Under- 
writers. This  list  (see  Table  XII,  page  80)  is  generally  accepted 
as  in  accordance  with  good  practice.  Smaller  than  No.  14  wire 
is  not  permitted  for  any  power  or  lighting  current,  except  in 
special  cases.  The  carrying  capacity  of  a  triple-braid  covered 
or  other  wire  without  rubber  is  greater  than  that  of  a  rubber- 
covered  wire,  on  account  of  deterioration  of  the  rubber  with 
heat.  However,  rubber-covered  wire  is  required  by  the  Under- 
writers in  certain  cases. 

Overhead  wiring  out  of  doors  should  be  strong  enough  to  with- 
stand wind  and  sleet,  and  good  practice  calls  for  a  No.  6  wire  as 
the  smallest  to  be  used  as  a  pole  line.  Larger  sizes  should  be 
used  for  larger  currents,  in  at  least  approximate  conformity  to  the 
ratings  of  the  National  Code,  even  though  it  be  where  the  Fire 
Underwriters  have  no  jurisdiction.  Underground  circuits,  in 
cables  and  conduits,  should  usually  have  not  smaller  than  No. 
8  wire,  and  should  conform  at  least  approximately  to  the 
National  Code. 

CONCLUSIONS 

After  finding  the  size  of  wire  required  for  allowable  line  drop, 
economy  and  safety,  the  largest  of  the  three  is  to  be  selected — 
for  obviously  we  cannot  exceed  the  maximum  voltage  drop,  just 
because  it  is  safe  and  economical;  nor  can  the  other  limitations 
be  disregarded.  If  the  requirements  for  voltage  drop  or  safety 
call  for  an  excessively  large  wire,  it  may  be  possible  to  obtain 
special  concessions  from  the  proper  authorities.  If  the  size 
required  for  economy  is  very  large,  it  may  not  be  possible  to  tie 
up  extra  capital,  even  if  it  is  economical  to  do  so.  In  any  of 
these  cases  it  is  well  to  consider  whether  a  higher  line  voltage  can 
be  used,  thereby  reducing  the  line  current. 


80 


ELECTRICAL  EQUIPMENT 


TABLE  XII.— DATA  ON  WIRES 


Size  of  wire  or  cable 

Weight  of  copper*  wire 
or  cable  in  pounds 
per  1,000ft. 

Safe  carrying    capac- 
ity of  copper*  wire  or 
cable  in  amperes 
(National  Elec.  Code) 

B.  &  S. 
or 
A.  W.  G. 
No. 

Area  in 
circular 
mils1 

Outside  diameter 
in  mils1 

Bare 

Triple 
braid* 

Bare 

Triple 
braid 
weather- 
proof4 

Rubber 
insula- 
tion 

Other 
insula- 
tions 

Stranded 

conductors 
1,000,000 
950,000 
900,000 
850,000 
800,000 
750,000 

1,152 
1,123 
1,093 
1,062 
1,031 
998 

1  451 
1  300 

3,090. 
2,930. 
2,780. 
2,620. 
2,470. 
2,320. 

3,478 
2,6i5 

650 
600 
550 

1,000 
920 

840 

700,000 
650,000 
600,000 
550,000 
500,000 
450,000 

964 
929 
893 
855 
814 
772 

1  i<J6 

i  ios 

2,160. 
2,010. 
1,850. 
1,700. 
1,540. 
1,390. 

2,113 
1,781 

500 
450 
400 

760 
680 
600 

0000 
000 

400,000 
350,000 
300,000 
250,000 
212,000 
168,000 

728 
681 
630 
575 
528 
470 

1,020 

"936 
862 
785 
728 

1,240. 
1,080. 
926. 
772. 
653. 
518. 

1,445 

1,126 
937 
806 
655 

325 
275 

225 

175 

500 
400 

325 
275 

00 
0 
1 
2 
3 
4 

133,000 
106,000 
83,700 
66,400 
52,600 
41,700 

418 
373 
332 
292 
260 
232 

662 
605 
518 
440 

379 

411. 
326. 
258. 
205. 
163. 
129. 

515 
420 
328 
267 

i73 

150 
125 
100 
90 
80 
70 

225 
200 
150 
125 
100 
90 

5 

6 

7 
8 

33,100 
26,300 
20,800 
16,500 

206 
184 
164 
146 

327 
290 

102. 
81.00 
64.30 
51.00 

ii7 

75 

55 
50 

35 

80 
70 

50 

Solid 
0000 
000 
00 
0 
1 
2 

conductors 
212,000 
168,000 
133,000 
106,000 
83,700 
66,400 

460 
410 
365 
325 
289 
258 

660 
610 
560 
510 
445 
400 

641. 
508. 
403. 
319. 
253. 
201. 

758 
616 
485 
396 
310 
255 

225 
175 
150 
125 
100 
90 

325 
275 
225 
200 
150 
125 

3 

4 
5 
6 
7 
8 

52,600 
41,700 
33,100 
26,300 
20,800 
16,500 

229 
204 
182 
162 
144 
128 

346 
303 

264 

159. 
126. 
100. 
79.50 
63.00 
50.00 

ie4 
ii2 

75 

80 
70 
55 
50 

35 

100 
90 
80 
70 

50 

9 
10 
11 
12 
13 
14 

13,100 
10,400 
8,230 
6,530 
5,180 
4,110 

114 
102 
91 
81 
72 
64 

221 
200 
i82 

39.60 
31.40 
24.90 
19.80 
15.70 
12.40 

53 
35 
25 

25 
20 
15 

30 
25 
20 

15 
16 
17 
18 

3,260 
2,580 
2,050 
1,620 

57 
51 
45 
40 

169 

9.86 
7.82 
6.20 
4.92 

19 

6 
3 

10 
5 

D.C.  TRANSMISSION  AND  DISTRIBUTION        81 


FOR  ELECTRICAL  CONDUCTORS 


Resistance  of  annealed 
copper8  wire  or  cable  in 
ohms  per  1,000  ft.  of 
single  conductor  at 

Reactance  on  60  cy- 
cles,8 in  ohms  per  1,000 
ft.8  of  single  conduc- 
tor, with  spacing 
of 

List  price*  of  copper 
wires  and  cables  per 
per  1,000  ft.,  on  the 

Size 

25°CJ 
(=  77°F.) 

50°C. 
(.=  122°F.) 

1  in. 

12  in. 

15-ct.» 
base 

20-ct. 
base 

In  circular 
mils,  or  in 
A.  W.  G.  No. 

0.01077 
0.01134 
0.01197 
0.01267 
0.01346 
0.01437 

0.01181 
0.01243 
0.01312 
0.01389 
0.01475 
0.01575 

0  .  0206 

0.0776 

1,945. 
1,860. 
,773. 
,688. 
,602. 
,513. 

2,418. 
2,310. 
2,198. 
2,090. 
1,980. 
1,867. 

Stranded 
1,000,000 
950,000 
900,000 
850,000 
800,000 
750,000 

0.0216 
6!  0230 

0.0786 
0.0800 

0.01539 
0.01658 
0.01795 
0.0196 
0.0216 
0.0240 

0.01687 
0.01817 
0.01967 
0.0215 
0.0237 
0.0263 

0.0245 
6.'  0264 

6!  0289 
0.0300 

0.0815 
O'.  0834 

6!  0859 
0.0870 

,427. 
,340. 
,256. 
,170. 
,047. 
956. 

,758. 
,648. 
,539. 
,429. 
,284. 
,169. 

700,000 
650,000 
600,000 
550,000 
500,000 
450,000 

0.0270 
0.0308 
0  .  0360 
0.0431 
0.0509 
0.0642 

0.0296 
0.0338 
0.0395 
0.0472 
0.0558 
0.0704 

0.0312 
0.0327 
0.0345 
0.0365 
0.0384 
0.0411 

0.0882 
0.0897 
0.0915 
0.0935 
0.0954 
0.0981 

867. 
775. 
686. 
593. 
492. 
412. 

1,056. 
942. 
827. 
712. 
591. 
492. 

400,000 
350,000 
300,000 
250,000 
A.W.G.  0000 

poo 

0.0811 
0.1021 
0.1288 
0.1625 
0.205 
0.259 

0.0889 
0.1119 
0.1412 
0.1781 
0.225 
0.274 

0.0436 
0.0464 
0.0491 
0.0518 
0.0542 
0.0571 

0.1006 
0.1034 
0.1061 
0.1088 
0.1112 
0.1141 

344. 
289. 
227. 
170. 
144. 
122. 

407. 
338. 
267. 
202. 
168. 
141.70 

00 
0 
1 
2 
3 
4 

0.326 
0.410 
0.519 
0,654 

0.357 
0.449 
0.569 
0.717 

103.80 
90.10 

119.40 
102  .  70 

5 

6 

7 
8 

0.0624 

0.1194 

0.0677 

0.1247 

57.80 

65.60 

0  .  0500 
0.0630 
0.0795 
0.1001 
0.1263 
0.1593 

0  .  0548 
0.0691 
0.0871 
0.1099 
0.1385 
0.1747 

0.0394 
0.0421 
0.0447 
0.0474 
0.0501 
0.0527 

0.0964 
0.0991 
0.1017 
0.1043 
0.1070 
0.1097 

492.00 
412.00 
344.00 
289.00 
199.00 
149.00 

591.00 
492.00 
407.00 
338.00 
237.00 
179.00 

Solid 
0000 
000 
00 
0 
1 
2 

0.201 
0.253 
0.319 
0.403 
0.508 
0.641 

0.220 
0.278 
0.350 
0.442 
0.557 
0.702 

124.00 
102  .  70 
88.70 
75.60 

148.00 
121.80 
103.70 
87.50 

3 
4 
5 
6 

7 
8 

0.0580 

0.1150 

0.0633 
6!  0686 

0.1203 
6!i256 

47.70 

55.20 

0.808 
1.018 
1.284 
1.619 
2.04 
2.58 

0.885 
1.117 
1.408 
1.775 
2.24 
2.82 

41.40 
35.70 

27!  30 
2il76 

47.10 
40.50 

30!30 
23!  50 

9 
10 
11 
12 
13 
14 

0.0739 

0.1309 

0.0792 
'   6!  0846 

0.1362 

6!i4i6 

3.25 
4.09 
5.16 
6.51 

3.56 
4.49 
5.66 
7.14 

ie'.oo 

13.  80 

i7!20 

iiieo 

15 
16 
17 
18 

82  ELECTRICAL  EQUIPMENT 

NOTES   ON   TABLE   XII 

Note  1.  Units. — The  area  of  any  wire,  expressed  in  square  mils,  is  0.7854 
times  the  area  in  circular  mils.  The  area  expressed  in  square  inches  is 
0.7854  X  10~6  X  area  in  circular  mils. 

Diameter  expressed  in  inches  is  diameter  in  mils  X  10~3. 

Example.— The  area  of  No.  0000  bare  wire  in  square  inches  is 
0.7854  X  10~6  X  212,000  =  0.1665  sq.  in.  Its  diameter  is  0.46 
in. 

Note  2.  Outside  Diameter  of  Insulated  Wire. — These  diameters  are  only 
approximate,  because  different  manufacturers  provide  different  thicknesses 
of  insulation.  They  are  nearly  correct  also  for  slow-burning  weatherproof, 
and  slow-burning  Underwriters'  wires.  Single-  and  double-braid  weather- 
proof wires  are  a  little  smaller. 

Note  3.  The  weight  of  hard-drawn  bare  aluminum  wire  is  30.4  per  cent, 
of  that  of  the  same  size  of  annealed  copper  wire.  (Aluminum,  as  used  in 
American  practice,  is  nearly  always  hard  drawn.) 

Example. — No.  00  bare  stranded  aluminum  conductor  weighs 
30.4  X  411,  or  125  Ib.  per  1,000  ft. 

Note  4.  Weight  of  Insulated  Wire. — The  weights  given  for  triple- 
braid  weatherproof  wire  are  only  approximate.  For  a  first  approximation 
the  weights  of  wires  with  other  kinds  of  insulation  can  be  found  from  the 
following  relations: 


The  weight  of  the  insulating  covering  of 


Equals  the  weight  of  the  cover- 
ing of  triple-braid  weather- 
proof wire,  multiplied  by 


Double-braid  weatherproof. . . 
Slow-burning  weatherproof . . . 
Underwriters'  weatherproof. . . 
Single-braid  rubber-covered. . . 
Double-braid  rubber-covered. . 


2.0 
1.5 
1.5 
2.0 


Example.-  *?$$}? 

No.  4  triple-braid  weatherproof  stranded  copper  conductor 

weighs 173 

No.  4  bare  stranded  copper  conductor  weighs 129 


Insulation  of  this  conductor  weighs 44 

If  the  same  conductor  has  a  slow-burning  weatherproof  cover- 
ing, the  insulation  weighs  approximately  2  X  44  or  88 
As  before,  the  copper  weighs 129 

And  the  total  weight  is  approximately 217 

Note  5.     Safe  carrying  capacity  of  insulated   aluminum  wire  is  84  per 


D.C.  TRANSMISSION  AND  DISTRIBUTION        83 


cent,  of  the  safe  carrying  capacity  of  the  same  size  of  copper  wire,  with  the 
same  insulation. 

Example. — A  500,000-circ.  mil  rubber-covered  copper  conductor 
has  a  safe  carrying  capacity  of  400  amp.  A  corresponding 
aluminum  conductor  will  carry  safely  0.84  X  400,  or  336  amp. 

Note  6.  Resistance  of  Copper  and  Aluminum. — The  resistance  of  a 
hard-drawn  copper  or  hard-drawn  aluminum  conductor  can  be  found  by 
comparing  with  that  of  an  annealed  copper  conductor.  The  resistance  of 

Hard-drawn  copper  is  1.02  to  1.03      /  *"?  *•*.»?  an  a™ealed  C°Ppetr 
Hard-drawn  aluminum  is  1.64  conductor  of  the  same  d.mensions,  at 

[  the  same  temperature. 

Example. — A  1,000,000-circ.  mil  hard-drawn  aluminum  stranded 
conductor  has  1.64  X  0.01077  or  0.0176  ohm  per  1,000  ft.  at  25°C. 

Note  7.  Resistance  at  Other  Temperatures. — The  table  gives  resistances 
at  25°  and  50°C.  By  interpolation  or  extrapolation,  resistances  at  other 
temperatures  can  be  obtained. 

Example. — The  resistance  of  1,000  ft.  of  No.  10  annealed  copper 
wire  (not  stranded)  at  40°C.  is  1.018  +  ^  ~  jjf  (1.117  -  1.018)  or 

OL)  —   Zo 

1.077  ohms.     A  hard-drawn  aluminum  wire  of  the  same  dimensions 
and  temperature  is  1.077  X  1.64,  or  1.76  ohms. 

Note  8.  Line  reactance  is  proportional  to  the  frequency  and  to  the  length 
of  line.  It  also  depends  on  the  diameter  of  the  conductor  and  the  spacing 
between  conductors. 

Example. — A  transmission  line  of  300,000-circ.  mil  stranded 
conductors  is  3,000  ft.  long ;  the  distance  between  centers  of  conductors 
is  12  in.,  and  the  frequency  is  25  cycles.  The  reactance  given  in  the 
table,  for  12-in.  spacing,  is  0.0915,  and  therefore  for  the  required 

3,000 


length  and  frequency  it  is  j 


x  60  x  0-0915,  or  0.114  ohm. 


The  reactance  for  other  distances  between  conductors  is  obtained  from 
that  given  in  the  table  for  1-in.  spacing,  which  we  shall  call  X0. 


Distance,  center  to 
center,  between  con- 
ductors, in  inches 

Reactance  per  1,000 
ft.  on  60-cycles, 
in  ohms 

Distance,  center  to 
center,  between  con- 
ductors, in  inches 

Reactance  per  1,000 
ft.  on  60-cycles, 
in  ohms 

1 

*o 

15 

0.0621  +  Z0 

1H 

0.0093  +^o 

18 

0.0663  +XQ 

2 

0.0159  +X0 

24 

0.0729  +  X0 

3 

0.0252  +  Xo 

30 

0.0780  +  X0 

4 

0.0318  +  X0 

36 

0.0822  +  X0 

6 

0.0411  +  X. 

48 

0.0887  +  Xf 

9 

0.0504  +  X0 

60 

0.0939  +  Xo 

12 

0.0570  +  X0 

84  ELECTRICAL  EQUIPMENT 

Example. — To  find  the  reactance  of  a  No.  00  stranded  conductor 
1  mile  long,  on  25  cycles,  where  the  distance  between  centers  of  con- 
ductors is  15  in.  The  order  of  procedures  is  (1)  to  look  up  XQ  for  the 
given  size,  (2)  to  add  the  term  for  spacing,  and  (3)  to  multiply 
by  factors  for  frequency  and  length  of  line. 

X0  for  00  stranded  conductor  is 0. 0436  ohm. 

Spacing  term  for  15  in.  is 0 . 0621  ohm. 


Reactance  per  1,000  ft.  on  60  cycles  is 0 . 1057  ohm. 

5  280      25 
Required  reactance  is  77^  X  ™  X  0.1057  =  0.232  ohm. 

1,UUU         DU 

Note  9.  (a)  Cost  of  Copper  Wire. — The  list  price  is  subject  to  a  discount  of 
about  50  per  cent.,  depending  on  the  quantity  of  wire  that  is  purchased. 
List  prices  fluctuate  with  the  market,  and  quotations  are  made  at  some  par- 
ticular base  price,  which  is  arbitrarily  known  as  a  15-ct.,  16-ct.,  or  other 
base.  The  table  gives  the  15-ct.  and  20-ct.  base  price  list.  List  prices  on 
any  other  base  can  be  found  approximately  by  interpolating. 

Example. — To  find  the  list  price  of  No.  10  solid  conductor,  per  1,000 
ft.,  on  the  18-ct.  base.  The  prices  on  the  15-ct.  and  20-ct. 
bases  are  $35.70  and  $40.50  respectively.  On  the  18  ct.  base  it  is 

$35.70 +^-^^|  ($40.50  -  $35.70),    or    $38.58.     If   there    is    a 

discount  of  45  per  cent.,  the  cost  for  3,000  ft.  is  $38.58  X  (1.00  — 
0.45)  X  3,  or  $63.66. 

(6)  The  cost  of  labor  and  supplies  for  installing  varies  over  a  wide  range, 
depending  on  the  labor  market  and  the  kind  of  installation.  The  following 
give  only  a  first  approximation  of  the  cost  in  dollars  per  1000  ft.  of  wire. 
(D  =  diam.  of  bare  wire  in  mils.) 

For  pole  lines,  not  including  poles 0.035  D 

For  interior  knob  and  tube  work 10  +  0.035  D 

For  conduit  work,  not  including  conduits 15  +  0.05    D 


CHAPTER  XI 
A.C.  TRANSMISSION  AND  DISTRIBUTION1 

The  treatment  of  D.C.  transmission  and  distribution  systems 
applies  as  well  to  A.C.  systems  in  many  respects.  Certain 
points  are  taken  up  in  this  chapter  which  apply  only  to  A.C. 
circuits,  and  therefore  are  not  dealt  with  in  Chapter  X. 

VOLTAGE  DROP 

The  voltage  drop  in  an  A.C.  circuit  is  computed  as  in  D.C., 
except  for  three  considerations:  (1)  the  effect  of  line  reactance, 
(2)  the  effect  of  power  factor,  and  (3) 
the  difference  between  the  drop  in  Eo 

single-phase  and  polyphase  circuits. 

Reactance  in  a  Single -phase  Circuit. 
— We  shall  assume,  first,  that  the  cur- 


XI 


rent  is  in  phase  with  the  terminal  volt-    Fl°Q  loJ^^JJg?  dp°  Fdue 

age that    is,    that    the    load    is    at    100         I  is  line  current,  Et  is   termi- 

per  cent,  power  factor.     Fig.  31  is  a   ^^S 

vector  diagram  of  the  current  and  all    ] 

the  voltages.     The   current,  /,  is  in  phase  with  the  terminal 

voltage,  Et,  and  the  RI  drop  is  in  phase  with  7,  and  therefore 

with  Et.     The  reactance  drop  XI  leads  the  current  by  90°.     The 

generator  voltage,  Eg,  is  the  hypotenuse,  so  that 

Eg*  =  (Et  +  my  +  (xiy  (3) 

If  XI  is  small  compared  with  Eg,  the  square  is  still  smaller  in  its 
effect;  if  XI  is  not  over  10  per  cent,  of  Eg,  which  is  usually  the 
case,  the  error  caused  by  neglecting  it  does  not  exceed  0.5  per 
cent.  We  have  then  the  simple  approximate  relationship  be- 
tween Eg  and  Et, 

Ea  =  Et  +  RI,   or  EQ  -  Et  =  RI  (4) 

which  is  the  same  as  for  a  D.C.  circuit.     That  is,  unless  the 

1  See  references  at  the  beginning  of  Chapter  X,  p.  71. 

85 


86  ELECTRICAL  EQUIPMENT 

product  XI  is  excessively  large  compared  with  the  line  voltage,  the 
voltage  drop  can  be  calculated  just  as  on  D.C.  circuits,  as  RI  drop, 
in  case  the  load  current  is  at  unity  power  factor. 

Example.  —  Compare  the  results  of  equations  (3)  and  (4),  in  case  of  a 
220-volt  terminal  voltage,  where  the  current  output  of  the  line  is  at  unity 
power  factor,  the  RI  drop  is  11  volts,  and  XI  is  15  volts,  or  about  7  per  cent. 
of  the  line  voltage. 

According  to  equation  (3)  According  to  equation  (4) 

(Et  +  RIY  =  (220  +  II)2  =  53,361 
(XIY  =          152          =       225 
Egz  =  53,586 

Eg    =  231.51     Eg  =  Et  +  RI  =  220  +  11  or  231 

Introducing  an  error  of  0.2  per  cent. 

Power  Factor  on  Single  -phase.  —  The  condition  is  different 
from  the  foregoing  at  any  other  power  factor.  Let  the  current 


FIG.  32.  —  Voltage  drop  due  FIG.  33.  —  The  components  of 

to  lagging  current.  voltage  drop,  with  the  same  con- 

ditions as  in  FIG.  32. 

0  is  the  angle  of  lag  of  the  cur-  Ip  is  power  component  of  current  (in 

rent.     Meaning  of  other  symbols         phase  with  the  voltage),  Iq  is  quadra- 
is  the  same  as  in  Fig.  31.  ture  component  of  current  (90°  from 

the  voltage). 

lag  by  an  angle  6  as  in  Fig.  32.  The  RI  drop  is  in  phase  with 
the  current,  and  the  XI  drop  leads  the  current  by  90°  ;  but  the 
reactance  has  a  much  greater  effect  than  at  100  per  cent,  power 
factor,  because  it  is  partly  in  phase  with  the  voltage.  The  rela- 
tion between  Eg  and  Et  can  be  found  by  considering  the  current 
I  in  its  two  components,  as  in  Fig.  33.  Ip  is  the  power  compo- 
nent (=  I  cos  8),  and  Iq  the  quadrature  component  (=  / 
sin  0).  The  current  is  represented  as  lagging  behind  the  voltage 
rather  than  leading,  because  a  lagging  current  is  more  often 
found  in  practice.  The  total  drop  is  that  due  to  both  of  these 
components.  We  must  take  into  account,  then,  the  drop  due  to 
RIP,  RIq,  XI  p,  and  XI  q.  In  each  case  the  RI  drop  is  in  phase 
with  its  current,  as  shown  in  the  diagram,  and  each  XI  drop 
leads  its  current  by  90°.  The  generator  voltage,  Eg,  is  the  hypote- 
nuse of  a  triangle  whose  base  is  equal  to  Et  +  RIP  +  XI  q,  and 
whose  vertical  component  is  XI  p  —  RIq.  The  relation  between 
Eg  and  Et  is  shown  by  the  expression 

Ef  =  (Et  +  RIP  +  XI  qy  +  (XIP  -  RIqy  (5) 


A.C.  TRANSMISSION  AND  DISTRIBUTION        87 

In  ordinary  cases,  the  hypotenuse  is  very  nearly  the  same  as 
the  base,  and 

Eg  =  Et  +  RIP  +  XI q,    or   Eg  -  Et  =  RIP+  XI q       (6) 

That  is,  unless  the  quantity  (XIP  —  RIq)  is  excessively  large,  the 
total  drop  may  be  computed  as  the  resistance  drop  of  the  power 
component  plus  the  reactance  drop  of  the  quadrature  component. 

It  should  be  noted  that  both  a  low  power  factor  and  a  consider- 
able reactance  are  necessary,  for  the  reactance  to  have  much 
effect  on  the  voltage. 

Consider  the  same  case  as  before,  except  that  the  power  factor  is  86.6 
per  cent.  According  to  equation  (5) 

(Et  +  RIP  +  XIqy  =  (220  +  11  X  0.866  +  15  X  0.5)2  =  56,188 
(XIp  -  RIqy       =  (15  X  0.866  -  11  X  0.5)2  =         42 

\Ett*  =  56,230 

Eg    =  237.11 

According  to  equation  (6) 

Eg  =  220  +  11  X  0.866  +  15  X  0.5       =       237.04 
Introducing  an  error  of  0.03  per  cent. 

If  the  current  is  leading  instead  of  lagging,  Iq  has  a  negative 
value. 

Considering  the  same  case  as  before,  according  to  equation  (5), 

(Et  +  Rip  +  XIq)  =  (220  +  11  X  0.866  -  15  X  0.5)2  =  49,300 

(RI9  -  XIPY  =  (  -  15  X  0.866  -  11  X  0.5)2        =         342 

Ea*  =  49,642 

Eg  =  223 

According  to  equation  (6), 

Eg    =  220  +  11  X  0.866  -  15  X  0.5       =        222 
Introducing  an  error  of  0.4  per  cent. 

Polyphase  Circuits. — In  a  three-phase  F-connected  motor  or 
other  apparatus,  as  in  Fig.  34 (a),  if  the  three  windings  are  alike 
and  the  voltages  of  lines  A,  B,  C  are  balanced  (that  is,  AB  = 
BC  =  CA),  the  point  0  is  at  neutral  voltage,  and  voltage  OA  = 
OB  =  OC.  If  this  apparatus  takes  current  at  100  per  cent,  power 
factor,  the  three  currents  in  a,  b,  c  will  be  respectively  in  phase 
with  voltages  OA,  OB,  OC.  If  these  circuits  take  lagging  cur- 
rents, they  lag  behind  the  respective  voltages  to  neutral,  or  if 
leading  currents  they  lead  the  same  voltages. 


88 


ELECTRICAL  EQUIPMENT 


In  a  delta-connected  motor,  Fig.  35 (a),  if  the  three  elements 
are  all  alike,  and  each  takes  current  at  100  per  cent,  power  factor, 
the  three  currents  are  in  phase  with  the  respective  voltages 
AB,  BC,  CA.  The  total  current  entering  through  lead  A  is  the 
vector  sum  of  currents  ab  and  ac,  and  is  represented  by  aa  in 
Fig.  35(6).  This  has  the  same  phase  relation  as  current  a  in  Fig. 


FIG.  34. — Y-connected  motor,  or  other  apparatus,   on  a  3-phase  circuit. 

(a)  Electrical  connections,     (b)  Vector  diagram  of  currents. 

34(6).  Similarly,  if  the  currents  bb  and  cc  were  shown,  they 
would  be  in  phase1  with  the  corresponding  currents,  b  and  c  in 
Fig.  34(6). 

We  shall  now  consider  the  voltage  drop  in  a  circuit  furnishing 
current  to  a  F-connected  motor  or  other  apparatus;  remembering 
that  if  it  were  delta-connected,  the  phase  relations  of  the  line 


FIG.  35. — Delta-connected  motor  or  other  apparatus,  on  a  3-phase  circuit. 

(a)  Electrical  connections,     (b)  Vector  diagram  of  currents,  showing  that  the  total  current 
taken  from  the  line  has  the  same  phase  relation  as  if  the  apparatus  were  Y-connected. 

current,  and  therefore  of  the  line  drop,  would  be  the  same  as 
with  the  F-connection.  We  shall  first  find  the  voltage  drop  in 
a  single  line.  In  Fig.  36,  /  is  the  current  in  line  A,  and  ENt 

1  Of  course,  if  each  phase  AB,  BC  and  CA  takes  a  leading  or  lagging  cur- 
rent, the  resultant  currents,  aa,  bb,  cc  are  shifted  through  the  same  angle  as 
their  components. 


A.C.  TRANSMISSION  AND  DISTRIBUTION        89 

is  the  voltage  to  neutral  at  the  end  of  the  line.  The  RI  drop  is 
in  phase  with  7,  and  XI  is  90°  ahead  of  I,  exactly  as  in  the  single- 
phase  diagram,  Figs.  32  and  33.  Adding  to  ENt  the  RI  and  the 
XI  drop,  we  obtain  the  voltage  to  neutral  at  the  generator,  ENg. 
The  voltages  of  lines  B  and  C  to  neutral  are  represented  by  dotted 
lines.  The  terminal  voltage  between  lines,  ELt)  is  shown,  be- 
tween the  conductors  A  and  B,  and  the  corresponding  generator 
voltage  between  lines  is  ELg.  There  is  the  usual  ratio  of  vol- 
tage-between-lines  to  volt  age-to-neutral;1  ELt  =  \/3ENt  and 


FIG.  36. — Vector  diagram  of  line  voltages  and  voltage  drop  in  a  three-phase 

circuit. 

I  is  current  in  line  A.  Ril  is  resistance  drop  in  line  A.  X\I  is  reactance  drop  in  line  A. 
ENt  is  terminal  voltage  between  A  and  neutral.  ENg  is  generator  voltage  between  A  and 
neutral.  ELt  is  terminal  voltage  between  A  and  B.  ELg  is  generator  voltage  between  A 
andB. 

ELg  =  \/3ENg)  so  that  the  decrease  in  line  voltage,  ELg  — 
ELI,  is  V3  times  the  decrease  in  voltage  to  neutral,  ENg  —  Em. 
By  comparing  with  equations  (4)  and  (6),  we  find  that  if  RI 
is  the  resistance  and  Xi  the  reactance  of  a  single  line,  the  decrease 
in  line  voltage  on  a  three-phase  circuit  at  100  per  cent,  power 
factor  is, 

ELg  -  ELt  =  V3#J  (7) 

and  at  other  power  factors, 

ELg  -  ELt  =  V3  (RJP  +  XJt)  (8) 

1  G.  264,  Fig.  263. 


90 


ELECTRICAL  EQUIPMENT 


within  the  allowable  error — assuming  that  the  terms  neglected 
in  equations  (4)  and  (6  )are  not  excessively  large. 

A  three-phase  220-volt,  60-cycle  motor  takes  100  amp.  per  lead  at  80  per 
cent,  power  factor,  lagging.  The  motor  is  150  ft.  from  the  bus,  and  the  leads 
are  No.  0  stranded  conductors,  spaced  6  in.  apart.  What  generator  voltage 
is  required  to  provide  the  rated  voltage  at  the  motor? 

From  Table  XII,  page  80,  #1  =  0.1021  X  150/1,000  =  0.01531- 

Xl  =  (0.0464  +  0.0411)  X  150/1,000=  0.01313- 

From  Table  XIII,     sin  6  =0.6. 

From  equation  (8),  ELg  =  220  +  V3  (0.01531  X  100  X  0.80  + 

0.01313  X  100  X  0.60)         =  223.5  volts. 

TABLE   XIII. — CORRESPONDING  VALUES  FOR  POWER  FACTOR  AND  SIN  6. 
(For  lower  power  factors,  interchange  the  two  rows.) 


P.F.(  =  cos  0)..  .  . 

1.00 
0.00 

0.950 
0.312 

0.900 
0.436 

0.866 
0.500 

0.800 
0.600 

0.750 
0.661 

0.707 
0.707 

Sin  0 

In  a  two-phase  four-wire  circuit,  the  voltage  drop  in  each 
phase  is  independent  of  the  other.  It  is  treated  exactly  as  in 
a  single-phase  system.  On  a  two-phase  three-wire  system,  the 
current  in  the  common  conductor  is  the  resultant  of  the  other 
two.  In  general  the  phase  relation  of  this  current  is  such  that 
it  unbalances  a  little  the  voltages  of  the  two  phases. 

ECONOMY  AND  SAFETY 

Computations  for  the  most  economical  size  of  wire,  and  the 
requirements  for  safety  are  the  same  as  for  a  D.C.  system. 


CONCLUSIONS 

If  the  size  of  wire  required  for  allowable  drop,  economy  or 
safety  is  excessively  large,  the  possibilities  of  improvement  are 
greater  than  in  case  of  a  D.C.  system. 

Voltage  Regulation. — To  avoid  large  decrease  of  terminal 
voltage  on  account  of  line  drop,  we  may  apply  apparatus  in  any 
one  of  several  ways  as  follows: 

Transformers,  making  possible  a  much  higher  transmission 
voltage. 

Auto-transformers,  making  possible  a  little  higher  transmis- 
tion  voltage.  (Auto-transformers  cost  less  than  transformers, 
if  the  ratio  of  transformation  is  small;  but  they  have  little  or  no 


A.C.  TRANSMISSION  AND  DISTRIBUTION        91 

advantage  over  transformers  if  the  ratio  exceeds  3:1.  See 
Chapter  VII,  page  47). 

Synchronous  motors,  raising  the  power  factor  and  thereby 
eliminating  the  reactance  drop. 

Less  space  between  conductors  (e.g.,  by  putting  them  in  a 
cable),  thereby  somewhat  reducing  the  reactance. 

Lower  frequency,  reducing  the  reactance. 

Automatic  voltage  regulator,  maintaining  constant  terminal 
voltage,  even  with  large  line  drop. 

Economy. — If  the  size  of  conductors  required  for  economy 
is  prohibitive,  transformers  or  auto-transformers  reduce  the 
loss  with  a  given  size  of  conductor,  inversely  as  the  square  of 
the  transmission  voltage,  and  a  synchronous  motor  or  a  syn- 
chronous converter  reduces  the  loss,  inversely  as  the  square  of 
the  ratio  of  the  new  to  the  old  power  factor  of  the  line  current. 

Safety. — If  the  size  of  conductor  is  excessive  for  carrying  the 
current  safely,  an  increase  in  voltage  should  be  considered.  The 
current,  and  therefore  the  size  of  conductor,  is  thereby  reduced. 
Thus,  an  increase  in  voltage  is  good  for  all  the  three  requisites: 
regulation,  economy,  and  safety. 


CHAPTER  XII 
DIRECT-CURRENT  GENERATORS1 

CHARACTERISTICS 

Three  kinds  of  data  are  important  to  know  about  the  behavior 
of  any  D.C.  generator:  (1)  The  regulation  curve;  (2)  the 
efficiency  curve;  and  (3)  the  load  rating. 


no 


25 


125 


50  75  100 

Percent  of  Full-load  Current 

FIG.  37. — Regulation  curves  of  shunt,   flat-compounded,   and  over-com- 
pounded generators. 

The  regulation  curve  shows  the  variation  of  voltage,  as  the 
current  output  is  increased  from  zero  to  an  overload.  All 
ordinary  D.C.  generators  are  shunt-  or  compound-wound.  Their 
regulation  is  as  illustrated  in  Fig.  37.  The  regulation  curve 
of  a  small2  shunt  generator  is  poor.  If  the  voltage  is  adjusted 
to  the  correct  value  at  light  load  by  the  field  rheostat,  it  drops 
off  badly  as  the  load  increases;  or  if  the  adjustment  is  made  at 

1  G.  Chapter  XIII,  Characteristics;  XVI,  Efficiency;  XXIV,  Operation. 
S.   8:144-156,  Characteristics;  8:177-199,  Weight,  Cost,  Connections; 

8:230-256,  Operation;  8:275,  Acceptance  Tests;  10:715-750,  Applications. 

A.  pp.  653-654,  Applications;  pp.  674-675,  Regulation  and  Efficiency; 
pp.  685-687,  Operation,  Cost,  Weight. 

See  references  to  Chapter  VIII,  p.  51,  for  Train-lighting  generators. 

2  Shunt  generators  with  capacities  of  1000  kw.  or  more  frequently  have 
fairly  good  regulation. 

92 


DIRECT -C  URRENT  GENERA  TORS  93 

a  heavy  load,  the  voltage  rises  excessively  when  the  load  drops. 
If  it  is  desirable  for  any  reason  to  use  small  shunt  generators, 
the  voltage  should  be  kept  constant  by  an  attendant  who  is 
ever-present,  or  by  an  automatic  voltage  regulator;  otherwise, 
the  apparatus  that  is  on  the  system  must  be  such  as  can  operate 
on  a  wide  range  of  voltage. 

In  the  compound  generator  the  voltage  at  full-load  is  equal 
to,  or  greater  or  less  than  at  no-load,  depending  on  the  number  of 
ampere-turns  in  the  series  field.  If  the  voltage  is  the  same  at 
full-load  as  no-load,  the  generator  is  said  to  be  flat-compounded. 
The  voltage  is  then  2  to  6  per  cent,  higher  at  points  between 
no-load  and  full- load.  If  the  voltage  is  higher  at  full-load  than 
at  no-load,  the  generator  is  said  to  be  overcompounded.  The 
purpose  of  overcompounding  is  to  compensate  for  line  drop  on 
long  lines,  which  have  relatively  high  resistances.  Another 
reason  for  overcompounding  is  that  even  on  lines  of  negligible 
drop  the  variation  of  voltage  with  load  at  heavy  loads  can  be 
reduced.  See  Fig.  37.  As  explained  later,  it  is  possible  to  re- 
duce the  compounding  of  a  generator,  by  putting  resistance  in 
parallel  with  the  series  field  winding.  This  reduces  the  voltage 
by  reducing  the  series  field  excitation  at  any  given  load.  It  is 
well  to  specify  fully  as  much  compounding  as  is  ever  likely  to  be 
required,  because  it  can  be  reduced  as  much  as  necessary,  but 
cannot  be  increased. 

The  efficiency  of  a  generator  is  low  at  very  light  loads,  because 
some  losses,  such  as  friction,  are  about  as  large  at  no-load  as  at 
full-load.  The  efficiency  increases  as  the  load  increases;  but 
it  drops  again  at  heavy  overloads,  because  some  losses,  such  as 
copper  loss,  vary  as  the  square  of  the  load.  Generators  are 
usually  designed  to  have  their  maximum  efficiency  at  75  to  100 
per  cent,  of  the  rated  full-load,  but  they  may  have  it  at  any 
desired  load.  Fig.  38  shows  the  characteristic  shape  of  a 
generator  efficiency  curve,  and  illustrates  the  fact  that  the  effi- 
ciency is  nearly  the  same,  through  a  rather  wide  range — on 
this  particular  machine  from  75  per  cent,  to  more  than  125  per 
cent,  of  full-load.  In  general  the  larger  the  capacity  of  the 
machine,  the  higher  is  the  maximum  value  of  the  efficiency. 
While  the  maximum  value  depends  on  the  design  and  operating 
conditions,  the  following  general  figures  are  nearly  correct  for 
any  good  D.C.  generator: 


94 


ELECTRICAL  EQUIPMENT 


Kilowatt 
rating 

10 

50 

1,000 


Maximum 
efficiency, 
per  cent. 

85 
90 

93  to  95 


These  values  also  hold  nearly  true  from  half -load  to  25  per  cent, 
overload. 

The  load  rating  of  a  generator  is  usually  the  load  that  it  will 
carry  continuously  without  a  temperature  rise  of  more  than  40° 
or  50°C.,1  and  without  excessive  sparking  at  the  commutator. 


250  500          750         1000        0250 

Kilowatt  Output 

FIG.  38. — Efficiency  curves  of  a  1,000  kw.  600  volt  B.C.  generator. 

PARALLEL  OPERATION 

Shunt  generators  operate  in  parallel  very  satisfactorily,  in 
that  no  special  provision  is  required  to  divide  the  load  between 
them,  and  there  is  very  little  danger  that  either  machine  will  be 
damaged  by  a  partial  short-circuit,  caused  by  unequal  excitation 
of  the  fields.  But  the  poor  voltage  regulation  of  small  machines 
makes  it  impracticable  to  use  them  where  any  approach  to  con- 
stancy of  voltage  is  required;  unless  there  is  some  provision  for 
voltage  regulation,  external  to  the  machine. 

Compound  Generators. — Adjustment  of  Compounding. — Any 
two  compound  generators  of  the  same  voltage  can  "be  operated 
in  parallel,  if  an  equalizer  is  provided,  and  a  suitable  resistance 

1  The  A.I.E.E.  standardization  rules,  1916,  permit  a  temperature  rise 
of  40°C.  as  measured  by  the  thermometer  at  the  hottest  observable  point, 
in  machines  having  cotton,  silk  and  similar  insulating  materials  that  are 
not  impregnated.  If  impregnated,  a  rise  of  50°C.  is  permitted.  The  best 
practice  is  to  have  the  insulation  impregnated. 


DIRECT-C  URRENT  GENERA  TORS  95 

is  permanently  connected,  if  necessary,  in  series1  with  the  series 
field  of  one  generator  to  adjust  the  compounding.  This  resist- 
ance should  be  adjusted  so  that  the  total  current  is  divided  be- 
tween the  two  generators  in  the  right  proportion  at  all  loads. 
It  should  be  used,  even  if  the  machines  have  the  same  compound- 
ing when  running  separately,  unless  the  RI  drop  in  the  series 
field  at  full-load  is  the  same  in  the  two  generators. 

If  it  is  desired  to  reduce  the  compounding  of  the  entire  plant, 
a  resistance  may  be  permanently  connected  in  parallel  with  the 
series  field  of  each  machine,  as  indicated  in  a  previous  paragraph ; 
but  as  the  series  fields  of  the  several  generators  are  in  parallel, 
any  shunting  resistance  is  in  parallel  with  all  the  series  fields, 
and  does  not  affect  one  field  much  more  than  another.  Thus, 
where  compound  generators  operate  in  parallel: 

Use  shunt  resistance  in  parallel  with  each  series  field,  to  change 
the  compounding  of  the  entire  plant. 

Use  series  resistance  to  change  compounding  of  one  machine 
relative  to  another.  Sometimes  a  shunt 
resistance  is  required  in  addition,  where 
the  generators  have  dissimilar  series  field 
or  armature  windings. 

Connections. — Fig.  1,  page  2,  shows  the  principal  connections 
of  two  compound  generators  operating  in  parallel.  The  equalizer 
is  not  necessarily  on  the  switchboard,  but  to  save  copper  and 
labor  (in  cases  where  the  leads  are  very  heavy)  it  is  run  under  the 
floor  from  machine  to  machine,  and  connected  to  switches, 
mounted  on  posts  or  "  equalizer  pedestals."  The  saving  is  con- 
siderable in  some  cases,  because  the  cross-section  of  the  equalizer 
lead  should  be  at  least  one-third  of  that  of  the  main  leads. 

Switch  and  Circuit-breaker. — Each  generator  should  have  a 
switch,  and  either  a  circuit-breaker  or  fuse,  completely  discon- 
necting at  least  one  pole  of  the  generator.  Usually  the  switches 
disconnect  both  poles,  even  if  there  is  only  a  single-pole  cir- 
cuit-breaker. If  there  is  an  equalizer  connection,  and  only  a 
single-pole  circuit-breaker  is  to  be  used,  the  breaker  should  be 
connected  to  the  armature  terminal  that  has  no  equalizer  lead. 

Meters. — There  should  be  an  ammeter  in  each  generator  cir- 
cuit, and  a  voltmeter  that  can  be  connected  to  a  plug  switch, 
across  any  generator,  before  that  generator  is  connected  to  the 

1  And  another,  if  necessary  in  parallel. 


96  ELECTRICAL  EQUIPMENT 

bus.     The  same  voltmeter  may  be  used  to  measure  bus  voltage, 
when  another  plug  switch  is  used. 

Field  Rheostat. — A  field  rheostat  is  always  to  be  inserted  in 
the  shunt  field  circuit,  for  voltage  regulation.  The  maximum 
current  of  the  shunt  field  winding  (when  the  resistance  of  the 
rheostat  is  cut  out)  is  1.5  to  3  per  cent,  of  the*  current  output  of 
the  generator.  The  rheostat  should  therefore  have  a  correspond- 
ing maximum  current  capacity;  and  for  a  good  control  of  the  vol- 
tage the  maximum  resistance  of  the  rheostat  should  be  about  the 
same  as  the  resistance  of  the  field  winding. 

COST,    WEIGHT   AND    AVAILABLE    SIZES 

Cost. — The  approximate  selling  prices  of  typical  550-volt 
D.C.  generators  of  usual  speeds  are  given  in  the  list  on  page 
185.  At  other  voltages  the  same  values  may  be  assumed,  at 
least  for  a  first  approximation. 

Available  Sizes. — Usually  a  machine  can  be  obtained  within 
25  or  50  per  cent,  of  any  desired  size.  The  exact  sizes  that  are 
available  are  different  in  different  lines  of  machines,  but  the  follow- 
ing is  a  suitable  list  of  sizes  in  kilowatts,  of  D.C.  generators: 
1,  2,  3,  5,  7%,  10,  12J^,  15,  20,  25,  35,  50,  60,  75,  100,  125,  150, 
200,  300,  400,  500,  600,  750,  1,000,  1,250,  1,500,  2,000,  2,500. 
Engine-driven  generators  are  usually  between  25  and  500  kw. 

NUMBER  AND  SIZE  OF  GENERATORS 

For  determining  the  number  of  generators,  and  the  size  of  each, 
we  may  proceed  as  follows: 

Kilowatt  Capacity  of  Plant. — First  refer  to  time-load  curves 
of  the  station,  if  they  are  available, 1  and  find  the  maximum  power 

1  If  time-load  curves  are  not  available,  take  the  total  connected  load 
(i.e.,  the  sum  total  of  all  power  that  would  be  furnished  if  all  machines  and 
lamps  were  in  operation  at  full-load  at  the  same  time)  and  multiply  by  the 
demand  factor,  which  is  the  ratio  of  maximum  actual  demand  to  the  total 
connected  load.  The  demand  factor  for  power  and  lighting,  for  machine 
shops  and  factories,  varies  through  a  wide  range;  but  its  usual  value  is  about 
55  per  cent.  For  power  consumers  in  general,  having  more  than  one  motor 
installed,  the  average  of  the  demand  factors  in  a  large  number  of  cases  has 
been  found  to  depend  on  the  total  horsepower  of  motors  installed.  The 
following  empirical  expression  indicates  what  the  demand  factor  is  likely 
to  be  for  any  given  installation: 

Demand  factor  =  40  per  cent.  X  (HP.  +  30)/(HP.  +  15) 
where  HP.  is  the  total  horsepower  of  the  motors  installed. 


DIRECT -C  URRENT  GENERA  TORS  97 

that  the  plant  is  regularly  required  to  deliver  for  any  considerable 
time — say  for  2  or  3  hr.  (for  loads  of  shorter  duration  see  the 
note  on  "Intermittent  Loads,"  page  183).  Taking  this  as  the  re- 
quired full-load  capacity  of  the  plant,  the  sum  of  the  rated  ca- 
pacities of  the  several  generators  must  be  at  least  as  much  as  this 
value.  The  next  paragraph  cites  conditions  such  that  the  total 
generator  capacity  must  exceed  this  value. 

Allowance  for  Accidents  and  Repairs. — When  one  genera- 
tor is  out  of  commission  for  any  reason,  the  other  machines  must 
be  able  to  carry  the  load  safely.  If  there  are  five  or  more  machines 
of  equal  size  in  the  station,  the  overload  when  one  is  shut  down 
is  not  over  25  per  cent.,  and  usually  each  generator  is  designed  to 
carry  that  overload  for  a  time;  but  if  there  are  less  than  five 
generators,  the  emergency  overload  is  more  than  25  per  cent., 
and  would  be  excessive,  if  it  is  carried  for  any  considerable  time. 
On  this  account,  the  sum  of  the  machine  ratings,  when  any  gen- 
erator is  omitted,  must  equal  at  least  80  per  cent,  of  the  station 
peak  load;  and  it  is  better  that  the  available  generator  capacity 
be  90  or  100  per  cent,  of  the  peak  load,  if  it  is  of  long  duration 
(see  p.  94  for  overload  ratings) .  It  is  good  policy,  in  general,  to 
overload  electrical  machines  as  little  as  practicable,  even  though 
they  seem  to  carry  the  overload  successfully,  because  there  is  a 
gradual  deterioration  of  the  insulation  with  excessive  heating. 

Number  of  Generators. — The  larger  the  generator,  the  less 
the  first  cost  per  kilowatt,  and  the  higher  the  efficiency  of  the 
engine  and  generator.  On  the  other  hand,  with  a  few  larger 
machines  the  investment  in  the  idle  machine  is  greater.  The 
best  number  of  machines  is  a  compromise  between  maximum 
efficiency  and  minimum  investment  in  the  spare  engine-generator 
set.  It  may  be  found  by  trial,  by  such  a  method  as  the  following : 

The  annual  cost  for  fixed  charges,  and  expenses  for  operating 
and  maintenance  are  estimated  for  a  suitable  combination  of,  say, 
two  generators,  and  then  repeated  for  a  suitable  combination  of 
three  generators;  and  the  estimates  are  if  necessary  continued, 
increasing  the  number  of  machines  until  the  minimum  total 
annual  cost  is  reached,  beyond  which  an  increase  in  the  number 
of  generators  results  in  an  increase  in  the  total  annual  cost. 

A  numerical  example  will  illustrate  the  method  of  computation.  Assume 
the  following  data: 

The  plant  is  required  to  deliver  2,000  kw.,  24  hr.  per  day,  280  days  per 
year. 
7 


ELECTRICAL  EQUIPMENT 


Fixed  charges  for  interest,  depreciation,  insurance  and  taxes  amount 
to  a  total  of  15  per  cent,  of  the  first  cost. 

The  only  first  cost  to  be  considered  is  that  for  engines  and  generators. 
No  other  costs  are  assumed  to  be  greatly  affected  by  changing  the 
number  of  generating  units. 

Cost  of  engines  and  generators  is  as  indicated  on  page  185. 

Operating  expense  per  kilowatt-hour  of  energy  delivered  to  the  switch- 
board, including  fuel,  attendance,  supplies  and  repairs,  but  not 
including  fixed  charges,  is  as  follows  for  various  numbers  and  sizes  of 
generating  units,  under  the  conditions  of  operation  of  this  plant: 

Number  of  generating  units 2  3  4         5         6 

Output  rating  of  each,  kilowatts    2,000      1,000      750      500      400 
Cost  in  cents  per  kilowatt-hour 
delivered,  for  operation  and  main- 
tenance,1   including   fuel,    labor, 
and  supplies 0.6          0.65    0.7    0.8      0.9 

From  the  above  data  we  derive  the  following : 

Since  the  plant  is  loaded  continuously,  the  total  capacity  must  be  2,000 
kw.,  when  one  machine  is  shut  down. 


Number  of  units  

2 

2,000 
$16,300 
$40,500 

3 

1,000 

$8,300 
$20,500 

4 

750 
$6,300 
$15,500 

5 

500 
$4,300 
$10,500 

6 

400 
$3,500 
$8,500 

Size  of  each  generator  in  kilowatts 
(see   "Sizes  Available,"  page 
96)  . 

First  cost  of  each  generator  
First  cost  of  each  engine  

Total  first  cost  of  each  unit 

$56,800 
$8,520 

6.7 
0.127 
0.6 

$28,800 
$4,320 

4.47 
0.097 
0.65 

$21,800 
$3,270 

3.782 
0.087 
0.7 

$14,800 
$2,220 

2.68 
0.083 
0.8 

$12,000 
$1,800 

2.24 
0.080 
0.9 

Annual  fixed  charges  for  each 
unit  at  15  per  cent 

Total  energy  delivered,  by  each 
unit,   per  year,  in  millions  of 
kilowatt-hours  

Fixed  charges  in  cents,  per  kilo- 
watt-hour 

Operating  cost  in  cents,  per  kilo- 
watt-hour 

Total  cost  in  cents,  per  kilowatt- 
hour 

0.727 

0.747 

0.787 

0.883 

0.980 

This  shows  that  in  spite  of  the  large  investment,  it  pays  to  install  two 
2,000-kw.  units.  A  combination  of  smaller  units  would  be  advantageous 
if  the  machines  were  loaded  only  8  hr.  or  less  instead  of  24  hr.  per  day. 

1  If  the  generator  is  driven  by  a  compound  steam  engine. 

2  Assume  that  with  this  combination  of  750-kw.  generators  all  but  one  is 
in  service  at  full-load  continuously.     This  is  not  quite  true,  because  the  joint 
capacity  of  three  machines  is  2,250  kw.,  whereas  only  2,000  kw.  is  the  con- 
tinuous output ;  but  in  actual  practice  usually  the  full-load  requirements  of 
the  plant  are  not  so  definitely  fixed,  and  there  is  an  advantage  in  the  increased 


DIRECT-CURRENT  GENERATORS  99 

capacity  of  the  generators.  If  this  is  unnecessarily  large,  the  plant  may 
consist  of  two  600-kw.  and  two  750-kw.  units,  which  would  carry  the  2,000- 
kw.  load  with  hardly  an  appreciable  overload,  when  any  one  generator  is 
shut  down.  Another  arrangement  would  be  to  have  three  750-kw.  units 
and  one  500-kw.  It  is  better,  however,  for  the  sake  of  fewer  repair  parts,  to 
have  all  the  units  of  the  same  size  rather  than  to  have  some  a  trifle  smaller 
than  the  others. 


CHAPTER  XIII 

ALTERNATING-CURRENT  GENERATORS1 

CLASSIFICATIONS 

Alternators  (alternating-current  generators)  may  be  variously 
classified  as  follows: 

Phases. — A  single-phase  generator  is  simpler  in  construction 
than  a  polyphase,  but  it  has  the  objections  (1)  that  the  armature 
conductors  cannot  be  placed  so  advantageously  for  economical 
design;2  and  (2)  that  single-phase  is  not  so  well  adapted  as  poly- 
phase to  most  of  the  A.C.  motor  applications.  On  account  of 
these  objections,  nearly  all  A.C.  power  is  generated  as  two-phase 
and  three-phase;  and  of  these  three-phase  is  the  more  common  on 
account  of  some  minor  advantages  in  distribution  and  use.3 

Connections  between  Phases. — Three-phase  generators  (which 
nearly  always  have  the  phases  connected  together  at  the  gen- 
erator) are  either  Y-  or  delta-connected,  the  F-connection  being 
preferred  (see  Fig.  39). 

Two-phase  generators  nearly  always  have  the  phases  indepen- 
dent, that  is,  insulated  from  each  other.  This  is  because  inter- 
connection is  unnecessary  for  good  operation  of  the  system;  and 
it  is  safer  to  insulate  the  phases  within  the  machine,  unless  there 
is  some  reason  for  interconnection  (see  Fig.  40 (a)).  If  a  neutral 
lead  is  brought  out,  for  grounding,  or  for  multiple-voltage  appli- 
cations, the  neutral  points  are  interconnected,  as  in  Fig.  40(6). 
If  the  generator  is  not  interconnected  as  in  Fig.  40(6),  it  may  be 
connected  to  a  two-phase  three- wire  circuit,  as  in  Fig.  40  (c). 

1  G.  Chapter  XXXI,  Construction  and  Connections;  XXXII,  Charac- 
teristics. 

S.  7:1-15,  Types;  7:87-91,  Operation;  7:146,  Weights;  10:269,  715- 
750,  Costs  and  Applications. 

A.  pp.  616-619,  Types  and  Ratings;  638,  Efficiency;  645,  Phase-connec- 
tions; 647-652,  Operation,  weight,  cost,  speed. 

2  If  a  single-phase  and  a  polyphase  machine  have  the  same  size  of  frame, 
and  run  at  the  same  speed,  the  kilovolt-ampere  capacity  of  the  single-phase 
is  from  60  to  70  per  cent,  of  that  of  the  polyphase.     See  G.  260. 

3  See  Chapter  III,  p.  17,  for  the  advantages  of  each  kind  of  system. 

100 


ALTERNATING-CURRENT  GENERATORS 


101 


Frequency  should  be  adapted  to  the  apparatus  loaded  on  the 
circuits  (see  Chapter  III).  In  the  United  States,  60  cycles  is 
the  most  common  frequency.  It  is  used  advantageously  for 
arc  and  incandescent  lighting,  induction  and  synchronous  motors, 


FIG.  39. — Three-phase  generators. 

(a)  Y-connected.     (b)  Delta-connected. 

synchronous  converters,  and  other  applications.  The  only 
other  standard  frequency  in  this  country  is  25  cycles ;  an  impor- 
tant use  for  it  is  in  driving  single-phase  railway  motors  and  slow- 
speed  induction  motors.1  In  purchasing  a  generator  it  is  of 


I  Ground  if       /  ,  \  ,     \ 

(a)  |   Required      W  (O) 

FIG.  40. — Two-phase  generators. 

(a)  Phases  independent,     (b)  Phases  interconnected,     (c)  Two-phase  three-wire  circuit. 

advantage  to  select  one  of  standard  frequency  if  possible,  be- 
cause by  following  a  standard  design,  the  cost  may  be  less,  the 
delivery  more  prompt,  and  operation  better. 


1  25-cycle  circuits  are  also  in  general  use  for  large  transmission,  and  dis- 
tribution systems,  where  power  is  required  for  railway  and  lighting  purposes. 


102  ELECTRICAL  EQUIPMENT 

Speed  and  Prime  Mover. — The  higher  the  speed  of  a  genera- 
tor or  motor,  the  more  power  it  can  develop,  until  it  reaches  the 
limit  of  safe  or  practicable  speed.  This  fact,  together  with  the 
advantage  of  high  efficiency  of  the  prime  mover,  is  causing  turbo- 
generator sets  to  be  introduced  more  and  more  exclusively  for 
all  steam-driven  A.C.  circuits  that  are  of  considerable  size.  The 
best  operating  speed  of  a  turbine  makes  it  necessary  that  the 
generator  have  either  two  or  four  poles.  For  60-cycles,  which  is 
the  usual  frequency,  turbo-generators  of  sizes  up  to  6,500  kva. 
usually  have  two  poles,  and  run  at  3,600  r.p.m.  Above  that 
capacity,  up  to  35,000  kva.,  they  have  four  poles  and  run  at 
1,800  r.p.m. 

Small  alternators  are  still  driven  by  reciprocating  engines,  in 
some  cases;  these  and  all  large  and  small  generators  driven  by 
waterwheels  and  gas  engines  run  at  slower  speeds — from  58  r.p.m.1 
for  large,  slow-speed  units,  to  900  r.p.m.  for  high-speed  generators 
suitable  for  certain  applications  with  waterwheels. 

Voltage.2 — There  is  no  essential  difference  between  high-  and 
low-voltage  alternators  except  the  insulation  of  the  armature 
conductors.  But  at  very  high  voltages  the  relative  amount  of 
space  required  for  insulation  is  large,  especially  in  small  genera- 
tors which  have  only  a  small  total  space  for  armature  windings. 
For  this  reason,  the  smaller  the  kilovolt-ampere  capacity,  the 
lower  the  voltage  for  which  it  pays  to  build  the  generator.  The 
largest  generators  are  rarely  made  to  operate  at  more  than  13,200 
volts.  A  very  common  generator  voltage  is  2,300  or  2,400 
volts.  It  is  sufficient  for  local  distribution  for  several  miles, 
without  excessive  line  drop,  and  at  good  efficiency.  Sometimes 
a  generator  is  wound  for  2,300  volts  with  a  delta-connection, 
and  insulated  well  enough  to  change  later  to  a  F-connection, 
which  gives  4,000  volts.  Alternators  at  higher  voltages,  al- 
though not  so  common,  can  be  furnished  as  required.  Lower 
voltages,  especially  220,  440  and  550  volts,  are  well  suited  to 
many  industrial  plants. 

Revolving  Field  and  Revolving  Armature. — The  old  form  of 
alternator  was  like  a  D.C.  generator,  in  that  it  had  a  revolving 
armature.  It  has  now  been  displaced,  except  for  a  few  of  the 
very  small  low-voltage  generators,  by  the  revolving  field.  It  is 

1  The  Keokuk  waterwheel  alternators  have  124  poles,  and  run  at  58  r.p.m. 
S.  7:14. 

2  See  Chapter  III. 


ALTERNATING-CURRENT  GENERATORS 


103 


easier  to  bring  the  high-tension  current  of  the  armature  through 
the  leads  from  the  stationary  winding  than  through  slip-rings, 
and  it  is  easier  to  mount  and  insulate  the  heavy  armature  wind- 
ing on  a  stationary  than  on  a  moving  structure. 

CHARACTERISTICS 

In  general  the  same  information  is  required  regarding  the 
operation  of  alternators  as  of  D.C.  generators;  and  some  addi- 
tional information  is  also  necessary: 

Regulation. — The  regulation  of  an  alternator  is  different  at 
different  power  factors,  as  illustrated  in  Fig.  41.  With  a  lagging 
current,  a  large  decrease  in  voltage  is  produced  by  bringing  the 


ctor,  Leading 


Armature  Current 

FIG.  41. — Regulation  curves  of  an  alternator. 

current  from  no-load  up  to  full-load.  With  current  at  100  per 
cent,  power  factor,  the  decrease  is  not  so  great,  and  with  a  lead- 
ing current,  the  voltage  decreases  still  less  or  it  may  actually 
increase,  as  is  shown  in  the  curve.  The  difference  between 
these  curves  is  easily  explained  when  we  consider  the  phase 
relation  between  induced  voltage  and  the  voltage  drop  in  the 
armature  due  to  impedance. 

The  efficiency  of  an  alternator  with  a  load  at  100  per  cent, 
power  factor  is  practically  the  same  as  for  a  D.C.  generator  of  the 
same  size.1  For  the  largest  sizes  of  alternators — from  5,000 

1  See  Chapter  XII,  p.  94,  Efficiency  curve,  and  table  of  maximum 
efficiencies. 


104  ELECTRICAL  EQUIPMENT 

kva.  up — the  maximum  efficiency  is  as  high  as  96  or  97  per 
cent. 

The  load  rating  of  an  alternator  is  based  on  temperature  rise, 
as  is  a  B.C.  generator  rating;1  but  it  should  be  expressed  in  kilo- 
volt-amperes  rather  than  in  kilowatts,  because  the  limiting  cur- 
rent output  is  no  greater  at  low  than  at  high  power  factor.2  The 
output  capacities  of  single-phase  and  polyphase  alternators  are 
as  follows: 

Single-phase,  kva.  =  kw./P.F.  =  #1/1,000 

where  E  is  the  voltage  between  terminals,  I  the  rated  current  of 
each  lead,  and  P.F.  the  power  factor  of  the  load. 

Two-phase,  kva.  =  kw./P.F.  =  2      X  #1/1,000. 

Three-phase,  kva.  =  kw./P.F.  =  \/3  X  #7/1,000. 

In  three-phase  generators,  the  current,  7,  must  always  be  under- 
stood as  the  current  per  phase  in  the  external  circuit,  and  the  vol- 
tage, E,  that  between  external  lines. 

The  engine  capacity  is  based  on  the  kilowatt,  rather  than  the 
kilo  volt-ampere  output,  after  allowing  for  generator  efficiency. 

REQUISITES  FOR  PLANT  OPERATION 

Regulation  of  Prime  Movers  and  Alternators.3 — The  speed 
regulation  of  two  engines  must  be  the  same  in  order  that 
two  alternators  in  parallel  shall  automatically  divide  the  load 
equally  between  them  when  it  fluctuates.  This  is  evident, 
when  we  consider  that,  except  for  a  little  hunting,4  alternators 
operating  in  parallel  and  having  the  same  number  of  poles  must 
have  exactly  the  same  speed.  If  engine  A  has  its  governor  set 
for  a  higher  speed  than  engine  B,  it  will  if  necessary  pull  engine 
B  along,  by  driving  generator  B  as  a  synchronous  motor.  In 
actual  operation,  if  the  two  engines  are  governed  to  have 
very  nearly  the  same  speeds,  engine  A  takes  a  little  the  larger 
part  of  the  load,  and  that  brings  its  speed  down  a  trifle,  accord- 
ing to  its  speed-regulation  curve,  until  the  two  speeds  are  equal. 

1  See  Chapter  XII,  p.  94. 

2  See  Chapter,  VII,  p.  94. 
8  G.  2&7,  288. 

4  In  hunting,  one  machine  speeds  up  temporarily,  so  that  it  leads  the  other 
by  a  small  fraction  of  a  cycle ;  then  it  slows  down  until  it  lags.  The  machines 
do  not  fall  out  of  step,  but  if  the  hunting  is  bad  it  produces  a  troublesome 
pulsation  of  the  current. 


ALTERNATING-CURRENT  GENERATORS         105 

If  the  steam  is  properly  controlled,  the  two  engines  may  be  made 
to  deliver  the  same  amounts  of  power. 

Let  us  now  consider  the  generators.  Just  as  the  engines  must 
have  exactly  the  same  speed,  the  alternators  must  have  exactly 
the  same  terminal  voltages,  neglecting  a  small  drop  in  the  leads 
to  the  buses.  The  power  outputs  of  the  two  engines  are  about  the 
same,  and  the  efficiencies  of  the  two  alternators  are  high,  so  that 
the  power  outputs  of  the  two  alternators  must  be  about  equal. 
The  current  relation  of  the  two  generators  depends  on  their 
load-characteristic  curves  (see  Fig.  41).  If  one  of  them  has  more 
of  a  drooping  characteristic  than  the  other,  it  will  necessarily 
deliver  a  current  leading  that  of  the  other  generator,  since  the 
terminal  voltages  of  the  two  must  be  equal. 

Even  if  the  generators  have  similar  load  characteristics,  when 
the  rheostats  are  set  differently  on  the  two,  the  generator  having 
the  weaker  field  will  take  the  leading  current. 

For  perfect  parallel  operation  of  alternators  at  all  loads, 
there  are,  then,  three  conditions:  (1)  that  the  prime  movers  have 
the  same  variation  of  speed  with  load;  (2)  that  the  generators 
have  similar  load-characteristic  curves;  and  (3)  that  the  field 
rheostats  be  so  adjusted  that  the  no-load  voltages  are  equal. 

Synchronizing. — Three  conditions  must  be  satisfied,  at 
least  approximately,  on  all  phases,  before  two  alternators  can 
safely  be  thrown  in  parallel:  (1)  The  voltages  must  be  equal; 
(2)  the  frequencies  must  be  the  same;  and  (3)  corresponding  cir- 
cuits of  the  alternators  must  be  in  phase.  The  voltage  is  con- 
trolled by  the  generator  field  rheostat,  and  measured  by  means 
of  a  voltmeter  that  can  be  connected  to  any  generator,  by  a  plug 
switch.1  The  frequency  and  phase  relation  are  controlled  by  the 
speed  of  the  motor  or  prime  mover,  and  are  indicated  by  lamps 
or  a  synchronism  indicator 2  connected  by  plug  switches  between 
the  two  machines.  After  the  machines  have  been  properly 
installed,  it  is  necessary  only  to  compare  voltages  on  one  phase 
and  to  synchronize  on  one  phase,  because  the  ratio  of  voltages, 
and  the  lagging  or  leading  are  the  same  on  one  phase  as  on 
another.  If  the  generator  voltage  is  more  than  110  or  220,  it  is 
customary  to  use  voltage  transformers,  having  a  secondary  of 
about  110  volts,  for  use  with  the  voltmeter,  lamps,  and  syn- 
chronism indicator.  Some  engineers  prefer  to  synchronize 

1  See  Chapter  XIX,  p.  159. 

2  Sometimes  called  a  synchronoscope.     See  Chapter  XIX,  p.  150. 


106  ELECTRICAL  EQUIPMENT 

between  the  buses  and  the  generator  that  is  being  started, 
instead  of  between  one  generator  and  another.  The  method 
is  essentially  the  same  as  that  just  explained,  except  that  syn- 
chronizing connections  are  always  made  as  if  the  buses  were 
the  running  machine. 

Connections. — The  principal  connections  of  two  three-phase 
alternators  operating  in  parallel  are  shown  in  Fig.  2,  page 
2.  In  small  plants  of  low  voltage,  the  buses  are  mounted  on 
or  near  the  switchboard;  but  in  large  high-tension  plants,  only 
the  small  secondary  circuits  of  the  current  and  voltage  trans- 
formers, illustrated  in  Fig.  3,  page  3,  and  some  or  all  of  the 
D.C.  circuits  are  brought  to  the  switchboard.  With  such  an 
arrangement  all  measurements  are  made  and  the  entire  plant 
is  controlled,  by  means  of  apparatus  on  the  switchboard;  but 
it  would  incur  unnecessary  expense  to  bring  the  principal  leads, 
as  well  as  the  secondary  circuits  to  the  board. 

Switch,  but  No  Circuit-breaker. — It  is  common  practice  to 
open  all  phases  of  each  generator  circuit  by  a  two-,  three-  or 
four-pole  switch.  Usually  an  oil  switch  is  used  for  this  purpose, 
but  for  a  small  generator  of  600  volts  or  less,  a  knife  switch  may 
be  used.  It  is  the  best  practice  to  omit  circuit-breakers  on  A.C. 
generator  circuits;  because  the  short-circuit  current  of  the  genera- 
tor is  not  so  high  as  to  be  dangerous  to  the  machine  for  a  short 
time,  and  it  is  better  not  to  disconnect  the  machine  automatic- 
ally when  a  heavy  current  occurs,  thereby  putting  the  total  load 
on  the  other  machines.  There  are  overload  circuit-breakers  in 
the  outgoing  feeders,  and  these  offer  sufficient  protection  to  the 
generators. 

Meters. — (1)  There  must  be  means  for  measuring  the  current 
in  each  phase.  This  usually  consists  of  one  ammeter  for  each 
generator,  and  a  switching  device  for  inserting  the  meter  in  any 
phase,  as  desired  (see  Chapter  XIX,  pages  160.  161).  Usually 
the  ammeter  is  connected  to  the  secondaries  of  current  trans- 
formers. (2)  There  must  be  a  voltmeter  and  synchronizing 
outfit  as  mentioned  above,  under  synchronizing.  Usually  only 
one  voltmeter  and  one  synchronism  indicator  are  necessary  for 
the  entire  station.  (3)  It  is  important  to  know  the  power  factor 
of  the  load  on  each  generator,  in  order  to  bring  it  as  near  unity 
as  possible.  For  this  a  power-factor  meter  is  valuable,  although 
the  power  factor  can  be  calculated  if  watts,  amperes  and  volts 
are  known.  (4)  Sometimes  a  wattmeter  is  preferable  to  a  power- 


ALTERNA  TING-C  URRENT  GENERA  TORS         107 

factor  meter,  especially  if  the  kilowatt  output  of  each  generator 
is  to  be  observed  closely.  Sometimes,  but  rarely,  both  a  power- 
factor  meter  and  a  wattmeter  are  used.  (5)  In  addition,  if 
permanent  records  are  to  be  kept,  graphic  and  integrating  meters 
may  be  included  as  mentioned  in  Chapter  XIX,  page  164. 

Means  for  Voltage  Regulation. — The  voltage  variation  of  an 
alternator  with  variation  of  current  and  power  factor  is  so  great 
that  some  means  must  be  provided,  outside  the  generator,  to 
obtain  even  approximately  constant  terminal  voltage.  This 
regulation  is  most  successfully  accomplished  by  the  automatic 
or  hand  control  of  the  generator  field  current,  either  directly,  or 
more  often  indirectly  through  the  control  of  the  exciter  field.1 

Excitation. — Arrangement  of  Exciters. — If  each  generator  does 
not  have  its  own  exciter,  it  is  good  practice  to  provide  two  or 
more  exciters  in  the  station,  of  sufficient  capacity  to  carry  the 
total  excitation  load  with  one  exciter  shut  down.  The  exciters 
are  usually  compound- wound,  and  arranged  so  that  if  desired 
they  may  be  operated  in  parallel.  They  may  be  either  motor- 
or  steam-driven.  Motor-driven  exciters  are  efficient  and  simple 
in  their  operation,  and  should  be  used  wherever  practicable; 
but  there  must  be  at  least  one  exciter  that  has  some  way  of 
running,  before  there  is  any  voltage  on  the  A.C.  buses.  Exciter 
connections  and  equipment  are  in  general  the  same  as  in  case  of 
other  D.C.  generators,  except  that  circuit-breakers  are  usually 
omitted,  special  switches  are  used  for  connecting  to  the  alternator 
fields,  and  the  voltage  regulator  is  usually  connected  to  the 
exciter-field  rheostats. 

Switches. — Ordinary  knife  switches  can  be  used  for  connect- 
ing the  exciters  to  the  exciter  buses,  provided  a  special  field  switch 
is  inserted  between  the  exciter  buses  and  each  alternator  field. 
This  field  switch  must  be  so  constructed  that  even  when  it  is 
partly  opened  the  current  in  the  generator  field  winding  can  con- 
tinue to  flow  until  it  has  gradually  died  away.  The  inductance 
of  the  field  winding  of  a  large  alternator  is  so  great  that,  if  such 
a  provision  were  not  made,  the  extremely  high  voltage  caused 
by  opening  the  circuit  suddenly  would  strain,  and  perhaps  punc- 
ture the  insulation  of  the  field  winding.  One  arrangement  of 
such  a  field  switch  is  shown  in  Fig.  42.  Besides  the  main  switch- 
contacts  there  is  an  extra  contact  that  is  closed  at  just  the  instant 
before  the  main  contacts  are  opened.  The  field  current  can 

1  See  Chapters  XIV  and  XVI,  for  means  for  automatic  voltage  regulation. 


108 


ELECTRICAL  EQUIPMENT 


circulate  through  this  contact  and  a  resistance  box  in  series  with 
it,  until  the  energy  stored  in  the  field  is  dissipated. 

Voltage  and  Power-factor  Adjustments  by  Rheostats. — Where 
alternators  are  operating  in  parallel,  it  may  be  necessary  to 
change  the  voltage  of  a  single  generator,  in  order  to  change  the 
power  factor  on  that  machine.  But  in  order  to  raise  or  lower  the 
voltage  at  the  buses,  the  voltages  of  all  the  generators  should  be 
changed  together.  Each  of  these  changes  can  be  made  readily 
by  a  suitable  manipulation  of  the  exciter  and  alternator  rheostats. 
The  appropriate  current  capacities  and  maximum  resistances 

of  alternator-  and  exciter-field 
rheostats  can  be  obtained  from 
the  following: 

Size  of  Alternator-field  Rheostats. 
— It  is  found  in  practice  that  this 
rheostat  should  have  about  twice 
as  much  resistance  as  the  alter- 
nator-field winding.  Of  course, 
it  should  also  have  the  same  maxi- 
mum current  capacity  as  the  field 
winding.  The  alternator-field 

FIG.  42.— Alternator  field  con-  current,  and  the  resistance  of  the 

'  field  winding  can  be  found  ap- 
proximately  for  a  given  voltage 
of  excitation  from  the  fact  that  the  maximum  power  in  kilo- 
watts expended  in  exciting  a  turbo-generator  is  0.75  to  1.5  per 
cent,  of  the  kilo  volt-ampere  capacity  of  the  alternator;  the 
power  required  for  exciting  slower  speed  generators,  driven  by 
engines  and  waterwheels,  is  1.5  to  3  per  cent. 

Exciter-field  Rheostats. — For  exciters,  it  is  found  in  practice 
that  generally  the  resistance  of  the  field  rheostat  should  be  about 
the  same  as  that  of  the  exciter-field  winding.  The  exciter 
shunt-field  current  and  resistance  can  be  determined  from  the 
fact  that  the  maximum  field  current  is  1  to  3  per  cent,  of  the 
output  current  rating  of  the  exciter. 

Cost. — The  approximate  selling  price  can  be  obtained  from 
page  185. 


CHAPTER  XIV 
REGULATING  TRANSFORMERS 

Transformers  with  variable  ratios  are  used  to  regulate  current 
and  voltage.  For  the  best  operation,  this  regulation  should  be 
automatic,  but  there  are  cases  where  hand  regulation  is  allowable. 
There  are  two  distinct  kinds  of  regulating  transformers  in  common 
use:  the  constant-current  regulating  transformer,  and  the  induc- 
tion voltage  regulator. 

The  constant-current  regulating  transformer1  (or  const  ant- 
current  transformer — which  must  not  be  confused  with  the  current 
or  ammeter  transformer)  has  a  stationary  primary  winding  and  a 
movable  secondary.2  The  repulsive  force  between  the  two  wind- 
ings is  counterbalanced  by  a  weight;  when  the  current  becomes 
too  great,  it  predominates  over  the  weight,  and  moves  one  away 
from  the  other.  When  the  secondary  is  so  moved,  only  a  part 
of  the  primary  flux  links  with  the  secondary,  and  only  a  part  of 
the  secondary  flux  links  with  the  primary,  so  that  the  terminal 
voltage  of  the  secondary  automatically  decreases  enough  to 
bring  the  current  to  the  steady  value  for  which  it  is  counter- 
balanced. This  transformer  is  used  for  obtaining  constant 
current  for  series  arc-  and  tungsten-lighting  circuits,  when  the 
primary  is  connected  to  a  suitable  constant-potential  A.C.  cir- 
cuit. By  changing  the  counterweight,  the  value  of  the  secondary 
current  is  correspondingly  changed.  Customary  line  currents 
range  from  3%  to  10  amp.  The  connections  of  constant-current 
regulating  transformers  are  illustrated  in  Fig.  43.  The  primaries 
are  in  each  case  connected  to  three-phase  constant-potential  buses 
through  oil  switches.  Each  transformer  circuit  is  essentially 
single-phase,  on  both  primary  and  secondary,  even  though 
connections  are  made  to  a  three-phase  bus.  The  complete 
secondary  circuit  of  one  of  the  transformers  is  shown.  The 
others  have  similar  circuits  which  are  omitted  from  the 
diagram. 

Directly  across  the  secondary  is  either  a  plug,  or  an  oil  switch, 
PI,  by  which  it  can  be  short-circuited.  Such  a  short-circuit  does 

1  G.  295,  S.6:  9,  162-172.     Compare  6:  249-252;  A.  p.  1606. 

2  Or  stationary  secondary  and  movable  primary. 

109 


110 


ELECTRICAL  EQUIPMENT 


no  harm,  as  noted  above,  but  causes  the  secondary  to  move  away 
from  the  primary,  so  that  it  is  taking  practically  no  power  from 
the  primary.  This  plug  should  be  inserted  before  the  oil  switch 
in  the  primary  is  opened. 

Two  other  plug  or  oil  switches, P^P^  are  in  series  with  the  sec- 
ondary line,  connecting  the  transformer  to  the  outside  circuit. 
If  there  are  several  transformers  and  several  outside  circuits, 
sometimes  the  switches  are  arranged  so  that  any  transformer 
can  be  connected  to  any  circuit.  An  ammeter  is  in  series 
with  the  secondary,  to  indicate  whether  the  current  remains 
at  the  right  value.  If  it  is  too  high  or  too  low,  it  is  brought  to 

2200  Volt  Buses 


3  Phase  Alternators 


FIG.  43. — Constant-current  regulating  transformer  with  primary  con- 
nected to  constant  voltage  buses  and  secondary  connected  to  a  series-light- 
ing circuit. 

the  right  value  by  adjusting  the  counterweight.  In  the  outside 
circuit,  all  the  lamps  are  connected  in  series.  These  may  be 
either  arc  or  tungsten  lamps;  the  present  tendency  is  toward 
tungsten  lamps. 

If  one  of  these  transformers  is  to  operate  from  2,200-volt  buses,  to  light 
a  maximum  of  seventy-five  100-watt,  4-amp.  lamps,  assuming  that  line 
drop  is  negligible, 

Transformer  secondary  ampere  capacity  =4  =4 

Transformer  secondary  kilovolt-ampere  capacity  =  75/1,000  X  100  =  7.5 
Transformer  secondary  voltage  capacity  =  7.5  X  1,000/4    =  1,875 

Transformer  primary  voltage  capacity  =  2,200  =  2,200 

Transformer  primary  kilovolt-ampere  capacity     =  approximately  7.5 

Transformer  primary  ampere  capacity  =      approximately  7.5  X 

1,000/2,200  =  3.4 

As  the  number  of  lamps  decreases,  the  secondary  voltage  approaches  zero. 


REGULATING  TRANSFORMERS 


111 


Constant-current  A.C.  circuits  are  best  suited  for  use  where 
small  amounts  of  single-phase  power  are  required  at  low  voltage, 
at  points  along  an  extended  line,  or  over  a  large  area.  These 
restrictions  practically  limit  constant-current  A.C.  circuits  to 
series-lighting  systems  in  streets,  parks,  railroad  and  factory 
yards,  and  large  buildings.  They  are  admirably  adapted  to 
such  systems — especially  for  tungsten  lighting,  because  the  best 
efficiency  of  tungsten  lamps  is  on  such  a  very  low  voltage  per 
lamp  as  can  be  applied  only  where  the  lamps  are  put  in  series. 
Furthermore,  the  candlepower  of  a  tungsten  lamp  does  not  de- 
crease as  much  with  aging  on  a  constant-current  as  on  a  constant 
potential  circuit. 

The  induction  voltage  regulator1  is  essentially  a  transformer 
in  which  the  primary  is  connected  across  the  buses,  and 


i 

i . 

1° 

i 


!c 


,'c 


Position  I  Position  2  Position  3 

FIG.  44. — Vector   diagram    of   voltages    of   polyphase   induction   voltage 

regulator. 

a  is  bus  voltage.  6  is  regulator  secondary  voltage,  c,  the  dotted  line,  is  the  resulting 
feeder  voltage.  In  position  1,  the  regulator  is  turned  nearly  to  the  position  of  maximum 
feeder  voltage.  In  position  2,  the  feeder  voltage  is  very  little  more  than  the  bus  voltage. 
In  position  3,  the  feeder  voltage  is  much  less  than  the  bus  voltage. 

the  secondary  in  series  with  the  line.  The  primary  is  made 
rotatable,  and  the  heavy  secondary  winding  is  stationary.  The 
voltage  induced  in  the  secondary  either  increases  or  decreases 
the  line  voltage,  by  an  amount  depending  on  the  relative  posi- 
tions of  primary  and  secondary.  These  regulators  are  made  both 
as  single-phase  and  as  polyphase.  In  the  polyphase  regulator 
there  is  a  primary  and  a  secondary  winding  for  each  phase,  so  that 
the  voltages  are  increased  or  decreased  on  all  phases  at  the  same 
time.  The  vector  diagram,  Fig.  44,  shows  how  voltage  regulation 
is  accomplished  on  a  polyphase  regulator.  In  position  1,  the 
bus  or  primary  voltage,  a,  and  secondary,  b,  are  nearly  in  phase; 
and  the  resultant  line  voltage,  c,  is  nearly  at  its  maximum.  In 
position  2,  the  secondary  is  90°  out  of  phase  with  the  primary, 
and  the  line  voltage,  c,  is  almost  the  same  as  the  bus  voltage. 
1  G.  308,  369;  S.6:  238-248;  24:  210-214;  A.  p.  1607. 


112 


ELECTRICAL  EQUIPMENT 


In  position  3  the  primary  and  secondary  are  nearly  in  opposition, 
and  the  voltage  of  the  line  is  smaller  than  that  of  the  bus. 

The  connections  of  a  polyphase  regulator  are  illustrated  in 
Fig.  45.  The  diagram  represents  the  regulator  as  located  at  the 
beginning  of  the  feeder,  with  its  primary  connected  through  an 
oil  circuit-breaker  to  the  power-station  buses.  This  is  according 
to  standard  practice.  The  three  primary  windings  of  the  regu- 
lator, a,  a,  a,  are  delta-connected,  and  the  three  secondaries  are 
in  series  with  the  three  conductors  of  the  outgoing  feeder.  The 
voltages  indicated  on  the  diagram  are  typical  of  regulator 
operation:  the  primary  voltage  is  2,300;  the  regulator  raises  it  to 
2,500,  and  line  drop  brings  it  down  to  2,200  at  the  end  of  the  line. 

2300  Volt  Buses 


End  of  Feeder 

FIG.  45. — Three-phase  induction  feeder  voltage  regulator  and  connected 

equipment. 

o  =  Primary  circuits,  between  lines.     &  =  Secondary  circuits,  in  series  with  lines. 

When  the  line  current  is  so  small  that  the  line  drop  is  negligible, 
the  feeder  terminal  voltage  would  rise  to  2,300,  except  for  the 
regulator;  but  the  regulator  is  turned  to  such  a  position  that  it 
decreases  instead  of  increases  the  voltage,  thereby  holding  it 
constant  at  2,200.  The  connections  for  a  single-phase  regulator 
are  the  same  as  for  one  phase  of  a  three-phase  regulator.  The 
number  of  turns  in  the  secondary  is  usually  5,  10,  or  20  per 
cent,  of  the  number  in  the  primary,  so  that  the  maximum  possible 
boosting  or  bucking  is  5, 10,  or  20  per  cent,  of  the  primary  voltage. 
The  regulators  can  be  operated  by  hand;  but  in  many  cases  it 
is  important  to  maintain  a  constant  voltage  without  keeping  an 


REGULATING  TRANSFORMERS  113 

operator  continually  watching  the  voltmeter.  A  relay  (some- 
times called  a  contact-making  voltmeter)  is  then  used,  which 
controls  the  regulator  and  automatically  raises  the  voltage, 
when  it  is  too  low  at  the  end  of  the  line.  The  method  of  accom- 
plishing this  is  described  in  Chapter  XVI. 

Voltage  regulators  fill  important  places  where  there  are  several 
feeders  of  different  lengths,  each  having  a  different  line  drop. 
It  is  not  possible  to  regulate  the  generator  voltage  for  all  these 
feeders,  because  when  the  line  drop  is  large  on  one  feeder  it  may 
be  small  on  another.  Thus  in  Fig.  45,  if  regulators  are  put  on 
feeders  Nos.  2  and  3,  the  terminal  voltages  of  those  feeders,  as 
well  as  of  No.  1,  are  maintained  constant. 


CHAPTER  XV 
INSTRUMENT  TRANSFORMERS1 

These  transformers  differ  from  all  the  foregoing,  in  that  their 
secondaries  connect  only  to  meters,  relays,  and  similar  apparatus, 
so  that  in  some  sense  they  furnish  no  useful  power.  There  are 
two  kinds  of  instrument  transformers — voltage  and  current 
transformers. 

1.  Voltage  transformers  (otherwise  known  as  shunt,  potential 
or  voltmeter  transformers)  are  the  same  in  form,  winding  and 
theory  of  operation  as  the  power  transformers  that  carry  the 
main  currents.  They  have  the  same  connections  in  their 
primaries  as  power  transformers  sometimes  have.2  But  they 
are  very  much  smaller — having  a  capacity  of  200  volt-amp,  or 
less — and  are  used  only  to  furnish  power  to  voltmeters,  and  the 
voltage  windings  of  wattmeters,  over-  and  undervoltage  relays, 
voltage-regulating  relays  and  the  like.  Besides  reducing  the 
voltage  to  a  value  that  is  convenient  for  use  with  the  various 
instruments,  they  insulate  the  instruments  from  the  high-tension 
circuit,  so  that  they  may  be  handled  safely. 

The  primaries  of  these  transformers  are  made  for  all  the  vol- 
tages that  are  used  in  practice.  The  secondaries  are  commonly 
wound  for  about  110  volts,  thereby  adapting  the  transformers 
for  use  with  ordinary  instruments. 

There  is  a  slight  variation  of  the  secondary  voltage,  dependent 
on  the  phase  and  amount  of  current  flowing  from  the  transformer 
to  the  meters,  but  a  good  transformer  holds  a  true  ratio  between 
primary  and  secondary  voltages,  within  a  fraction  of  1  per  cent., 

1  "Characteristics  and  Grouping  of  Current  Transformers,"  by  HAROLD 
W.  BROWN.     The  Electric  Journal,  vol.  VIII,  1911,  pp.  642,  1023,  1109. 
See  also  references  on  Chapter  XVI,  p.  121. 

S.  6:10,  182-192;  3:79,  103-105;  24:741,  Theory,  Errors,  Applications. 
S.  10:826,  827,  Costs.  A.  pp.  1638-1652,  Theory,  Errors,  Applications, 
Costs. 

2  See  Chapter  VII,  p.  49. 

114 


INSTRUMENT  TRANSFORMERS  115 

if  the  voltage  is  reasonably  high  but  not  above  the  rated  voltage, 
and  the  current  is  not  in  excess  of  the  rating. 

Based  on  this  accuracy  of  transformation,  it  is  rather  interest- 
ing to  see  how  fully  all  the  conditions  of  the  primary  are  repro- 
duced in  miniature  in  the  secondary.  Instead  of  winding  instru- 
ments at  great  expense  for  2,200  to  110,000  volts,  or  whatever 
the  line  voltage  may  be,  they  are  nearly  all  wound  for  the 
standard  110  volts,  and  the  task  of  the  voltage  transformers  is 
to  show  on  100  volts  all  the  fluctuations  and  the  phase  relations 
that  exist  on  the  high-voltage  line. 

The  connection  of  voltage  transformers  almost  invariably  used 
on  three-phase  circuits  is  that  shown  in  Fig.  46,  which  is  the  V- 
connection.  It  was  stated  in  Chapter  VII  that  this  connection 
is  not  good  for  ordinary  power  purposes;  but  for  instrument  use, 
the  power  handled  is  so  small,  and  the  secondary  voltage  so 


vwv/vwv 
', 


FIG.  46. — Grouping  of  voltage  trans-        FIG.   47. — Grouping  of    voltage 
formers  on  a  three-phase  circuit.  transformers   on   a   two-phase 

circuit. 


constant  even  with  an  unsymmetrical  arrangement  of  trans- 
formers, that  it  is  in  practically  universal  use.  The  advantage 
over  a  delta-connection  is  in  the  saving  in  cost  and  space,  and  in 
simplicity  of  connections.  The  three  secondary  voltages,  ab, 
be,  ca,  correspond  in  phase  and  amount  to  the  primary  voltages, 
AB,  BC,  CA.  On  two-phase  four- wire  circuits  there  are  four, 
instead  of  three  primary  leads — two  from  phase  A  to  one  trans- 
former, and  two  from  phase  B  to  the  other.  For  simplicity  of 
wiring,  the  secondary  connections  are  the  same  as  for  three-phase 
— requiring  only  three,  instead  of  four  wires.  This  is  illustrated 
in  Fig.  47. 

There  are  various  other  ways  of  grouping  voltage  transformers, 
but  they  are  rarely  used.  They  correspond  to  the  groups  of 
power  transformers,  shown  in  Fig.  23,  page  49. 


116 


ELECTRICAL  EQUIPMENT 


The  following  are  customary  primary  voltage  ratings  of  volt- 
age transformers: 

200  4,000  20,000 

400  5,000  25,000 

500  6,000  30,000 

600  10,000  40,000 

1,000  12,000  50,000 

2,000  15,000  60,000 

3,000 

Current  Transformers  (otherwise  called  Series  or  Ammeter 
Transformers). — The  construction  of  one  form  of  current  trans- 
former is  illustrated  in  Fig.  48.  The  laminated  punchings  are 
represented  as  circular;  they  may  be  of  any  convenient  shape. 
With  this  form  of  transformer,  the  primary  is  simply  a  straight 
conductor  passing  through  the  opening  in  the  punchings.  The 
alternating  current  in  the  primary  induces  a  flux  in  the  iron 


Primary 


Laminations' 


Secondary 


FIG.  48. — One  kind  of  current 
transformer. 


(6) 

FIG.  49. — Conventional  repre- 
sentation of  current  trans- 
formers. 


(a)  Representation  of  a  single  transformer.    The  straight  line  represents  the  primary, 
and  the  zigzag  line  the  secondary.    To  distinguish  between  positive  and  negative  terminals 
the  secondary  lead  marked  S  is  the  one  from  which  current  flows,  when  the  primary  cur- 
rent is  entering  at  P. 

(b)  Two  current  transformers  on  two  of  the  conductors  of  a  three-phase  circuit. 

around  it;  the  secondary  is  wound  around  the  iron,  so  that  the 
flux  in  the  iron  induces  an  e.m.f.  in  the  secondary.  Only  a  few 
turns  are  shown;  a  much  larger  number  are  actually  required. 
More  primary  turns  are  obtained  if  necessary  by  passing  the 
primary  conductor  several  times  through  the  opening.  The 
number  of  turns  required  is  determined  by  the  design  of  the 
transformer. 

A  current  transformer  may  be  represented  diagrammatically 
as  in  Fig.  49,  where  the  straight  line  represents  the  primary  and 
the  zigzag  line  the  secondary.  The  significance  of  this  repre- 
sentation of  transformers  becomes  at  once  apparent  from  the 
description  of  the  transformer  in  the  preceding  paragraph. 


INSTRUMENT  TRANSFORMERS  117 

Current  transformers  are  sometimes  represented  in  diagrams  in 
several  other  ways,  but  they  are  no  simpler  or  clearer  than  this. 

The  same  relationships  hold  in  these  as  in  other  transformers — 
the  voltage  ratio  is  as  the  number  of  turns,  and  the  current  ratio 
inversely  as  the  turns — but  in  these  transformers  the  primary 
is  put  in  series  with  the  line,  and  the  primary  voltage  is  merely 
a  small  line  drop,  whereas  on  power  transformers,  voltage  trans- 
formers and  all  others  that  we  have  considered,  the  primary 
is  connected  across  the  line.  If  the  secondary  of  an  ordi- 
nary transformer  is  short-circuited,  an  excessive  current  flows  in 
the  primary  and  in  the  secondary;  but  if  a  current  transformer 
is  short-circuited,  the  primary  current  cannot  be  more  than  is 
flowing  in  the  main  line.  The  effect  of  the  short-circuit  is 
merely  to  reduce  the  voltage  across  the  secondary,  and  so  across 
the  primary.  This  makes  the  transformer  work  all  the  better; 
because  a  high  voltage  across  the  primary  calls  for  a  large  excit- 
ing current,  and  therefore  introduces  errors  in  ratio  and  phase 
relation.  A  short-circuit  eliminates  these  errors.  On  account 
of  these  possible  errors,  if  the  charge  for  electrical  energy  is 
based  on  watt-hour  meter  indications,  the  watt-hour  meter 
should  not  be  connected  to  the  current  transformers  that  actuate 
relays,  or  circuit-breaker  trip-coils,  unless  it  has  been  found  that 
no  excessive  error  is  produced  by  the  extra  apparatus.  The 
added  apparatus  sometimes  has  sufficient  impedance  to  introduce 
an  error,  decreasing  the  meter  readings  by  2  or  3  per  cent.1  Con- 
sider what  this  means,  in  case  a  customer  is  purchasing  5,000 
kw.,  10  hr.  per  day,  25  days  per  month,  at  1  ct.  per  kw.-hr., 
The  error  in  the  meter  indication  reduces  the  bill  by  2  per  cent., 
which  amounts  to  $3,000  per  year,  and  would  be  enough  to  pay 
for  the  extra  pair  of  transformers  for  the  polyphase  circuit  about 
once  in  3  or  4  days ! 

If  the  current  transformers  are  already  installed,  it  is  of  more 
than  passing  interest  to  make  a  test  on  this  point  as  follows: 
see  that  an  indicating  wattmeter  is  in  the  circuit,  and  connect 
a  knife  switch  around  the  trip-coil,  or  any  other  apparatus  that 
may  be  under  suspicion.  If  it  is  a  polyphase  circuit,  and  there 
are  two  or  more  trip-coils  or  relays,  a  two-  or  three-pole  switch 
should  be  used,  one  pole  being  connected  to  short-circuit  each 
trip-coil  or  relay.  When  the  load  is  steady,  at  about  normal 
load  and  power  factor,  open  and  close  the  switch,  taking  several 

1  See  Figs.  89  to  93,  pp.  156  to  158. 


118  ELECTRICAL  EQUIPMENT 

good  readings  with  the  switch  alternately  open  and  closed. 
It  is  easy  to  estimate  from  these  figures  whether  it  would  pay 
to  install  an  extra  pair  of  transformers,  in  order  to  obtain  a  true 
reading  of  the  watt-hour  meter.  Of  course,  such  an  error  may 
not  be  excessive  for  an  ammeter  or  even  a  wattmeter,  as  for 
a  watt-hour  meter,  because  so  great  accuracy  is  not  usually 
required.  For  a  relay,  it  is  well  within  the  allowable  limit  of 
error. 

If  the  secondary  of  an  ordinary  transformer  is  open-circuited, 
there  is  only  a  small  exciting  current  in  the  primary.  But  in 
a  current  transformer  the  primary  is  the  main-line  current,  and 
if  the  secondary  is  open-circuited,  all  the  main-line  current  acts 
as  an  exciting  current,  and  the  voltage  (primary  and  secondary) 
goes  up  to  many  times  its  value  under  normal  operation.  For  this 
reason  an  open  circuit  in  the  secondary  of  a  current  transformer 
is  dangerous.  There  have  been  conditions  such  that  death  has 
resulted  from  touching  the  two  terminals  of  the  secondary  of  an 
open-circuited  current  transformer,  whereas  in  normal  operation 
the  voltage  across  the  secondary  is  usually  from  1  to  10  volts. 
Even  if  the  transformers  are  so  placed  that  there  is  no  danger  to 
life,  the  voltage  may  still  be  high  enough  to  destroy  the  insula- 
tion, or  the  iron  loss  at  high  voltage  may  be  enough  to  burn  out 
the  transformer.  The  current  transformer  secondary  should 

always  be  short-circuited  when  not 
in  regular  service,  if  the  primary 
is  carrying  any  current.  Switches 

-Wattmeter  .  . 

for  this  purpose  are  described  in 
Chapter  XIX. 

Another  point  of  difference  be- 
tween these  and  the  ordinary 
transformers  is  in  the  secondary 

FIG.  50.— The  right  way  to    connections.      Lamps,    motors,    or 
connect  instruments  to  a  current 
transformer.  any  other  pieces  of  apparatus  are 

usually    put    in    parallel    on    the 

ordinary  transformer;  but  all  instruments  on  the  secondary 
of  a  current  transformer  must  be  in  series.  This  is  obvious 
because  otherwise  the  current  would  be  divided  between  the 
several  instruments;  no  one  of  them  would  have  all  the  current 
of  the  transformer.  The  right  connections  are  illustrated  in 
Fig.  50,  where  an  ammeter,  the  current  element  of  a  wattmeter, 
and  the  coil  of  a  relay  are  connected  in  series  to  a  current 


INSTRUMENT  TRANSFORMERS 


119 


transformer.     In  Fig.  51,  are  illustrated  three  of  the  many  wrong 
ways  of  connecting. 


FIG.  51. — Three  wrong  ways  of  connecting  the  same  instruments  as  in 
Fig.  50  to  a  current  transformer. 

Transformers  are  grouped,  on  polyphase  circuits,  so  as  to 
obtain  indications  on  all  phases.  One  arrangement  of  current 
transformers  is  shown  in  Fig.  52,  which  represents  a  three-phase 
circuit,  in  which  a  current  transformer  is  put  on  each  of  the 
lines,  A,  B,  C;  and  meters,  a,  6,  c  are  in  the  secondaries  of  the 
respective  lines.  The  three  secondary  circuits  are  combined 
in  the  common  return  wire,  r,  instead  of  coming  back  inde- 
pendently to  their,  respective  transformers. 

A    BAB' 


A     B 


ABC 


FIG.  52.— Three 
current  transformers 
and  connections,  on 
a  three-phase  circuit. 


FIG.    53. — Two 

current  transformers 
and  connections  on  a 
three-phase  circuit. 


2-Phase 

FIG.  54.— Two 
current  transformers 
and  connections  on  a 
two-phase  circuit. 


There  is  another  grouping  for  a  three-wire  ungrounded  cir- 
cuit, as  illustrated  in  Fig.  53,  which  saves  one  transformer.  The 
two  transformers  have  the  same  ratio.  The  secondary  currents, 
a  and  c,  are  in  phase  with  the  primary,  A  and  C;  and  b,  which  is 


120  ELECTRICAL  EQUIPMENT 

the  resultant  of  a  and  c,  is  proportional  to  and  in  phase  with  B, 
which  is  the  resultant  of  A  and  C.  This  grouping  does  not  give 
correct  indications  for  instruments  in  line  b,  if  the  primary  circin't 
has  such  connections  that  there  is  any  primary  current  returning 
through  ground  or  any  other  conductor.  It  is  standard  practice 
to  use  this  grouping,  where  there  is  not  likely  to  be  any  ground 
return  current,  and  there  is  no  neutral  wire. 

On  a  two-phase  system  such  as  is  shown  in  Fig.  54  there  are 
four  wires,  and  the  two  phases  are  entirely  insulated  from  each 
other.  Wires  A  and  A'  carry  the  current  for  phase  A,  and  B 
and  Bf  for  phase  B.  The  current  must  be  the  same  in  A'  as  in 
A,  and  the  same  in  B'  as  in  B;  so  that  only  two  current  trans- 
formers are  required  to  indicate  the  currents  in  the  four  wires. 

The  following  are  customary  primary  current  ratings  of  current 
transformers : 

5  50  250 

10  60  300 

15  80  450 

20  100  500 

25  120  600 

30  160  800 

40  200  1,000  amp. 

Advantages  of  Using  Instrument  Transformers. — Several 
things  are  accomplished  by  this  use  of  current  and  voltage  trans- 
formers: (1)  Both  the  current  and  the  voltage  instruments  are 
insulated  from  the  high-tension  line,  so  they  are  safe  to  handle; 
(2)  the  voltage  elements  do  not  require  the  high-voltage  winding, 
nor  the  current  elements  the  large  current  winding,  which  would 
be  expensive  in  some  cases;  and  (3)  all  current  instruments  can 
be  made  for  a  standard  current  of  5  amp.,  and  all  voltage  instru- 
ments for  a  standard  voltage  of  110  volts,  thereby  standardizing 
the  manufacture  of  instruments  and  reducing  their  cost.  Set 
over  against  these  advantages  is  the  cost  of  the  transformers, 
but  in  the  majority  of  cases  it  is  better  to  use  the  transformers. 


CHAPTER  XVI 
CONTROLLING  AND  REGULATING  EQUIPMENT1 

This  chapter  treats  of  the  automatic  and  manually  operated 
apparatus  that  is  used  to  adapt  the  several  circuits  to  their  varied 
requirements.  It  does  not  include  automatic  circuit-breaking 
apparatus,  which  is  discussed  in  Chapter  XVII,  nor  measuring 
and  indicating  apparatus  covered  in  Chapter  XIX.  The  pres- 
ent discussion  is  taken  up  under  the  following  headings :  (1)  Cir- 
cuit-opening and  closing  equipment,  (2)  rheostats  for  controlling 
motors  and  generators;  and  (3)  automatic  regulating  equipment. 

CIRCUIT-OPENING  AND  CLOSING  EQUIPMENT 

Knife  Switches. — On  D.C.  circuits,  knife  switches  can  be  used 
for  all  ordinary  voltages.  A  knife  switch  may  be  omitted  if  a 
two-pole  circuit-breaker  is  arranged  so  that  one  pole  can  be  closed 
at  a  time.  Otherwise  it  is  not  safe  to  use  a  two-pole  breaker  as 
a  switch,  because  there  may  be  a  short-circuit  on  the  line,  while 
the  breaker  is  being  held  in  by  hand.  A  single-pole  switch  is 
used  on  a  single-polarity  system  (in  which  the  opposite  side  of 
all  equipment  is  grounded) ;  usually  a  two-pole  switch  is  used  on 
a  double-polarity  system  (in  which  neither  side  is  grounded); 
and  on  a  three-wire  system  either  a  two-  or  three-pole  switch  is 
used,  depending  on  the  application. 

Knife  switches  are  used  on  low-voltage  single-phase  and  poly- 
phase circuits,  as  well  as  on  D.C.  But  on  voltages  above  550, 
and  even  on  lower  voltages,  oil  switches  are  often  used  instead  of 
knife  switches  on  A.C.  circuits. 

1  G.  Chapter  VI,  Rheostats;  Chapters  XIX,  XX,  XXI,  Switches,  Starters, 
Controllers;  Par.  276,  Regulators.  S.  10:  761-836,  Switching  Equipment; 
15:  429-467,  Motor  Control;  10:  751-760,  Regulators. 

A.  pp.  220-227,  1483-1503,  Circuit-breakers  and  Switches;  1363-1370, 
276-278,  Motor  Starters  and  Controllers;  1231-1240,  Rheostats;  1212-1215, 
Regulators. 

"Meter  and  Relay  Connections,"  by  HAROLD  W.  BROWN.  The  Electric 
Journal,  vol.  V,  1908,  pp.  260,  406,  460,  530,  597,  660,  725;  vol.  VI,  1909, 
pp.  47,  113,  173,  298,  430. 

121 


122  ELECTRICAL  EQUIPMENT 

A  knife  switch,  of  whatever  capacity,  should  be  rugged  in  its 
construction;  and  should  conform  to  the  requirements  of  the 
National  Electric  Code,  for  its  rated  current  and  voltage.  In 
prescribing  switches,  several  features  are  to  be  specified : 

1.  Number  of  Poles. — Usually  the  number  of  poles  is  made 
sufficient  so  that  a  single  switch  opens  the  entire  circuit,  e.g.,  a 
four-pole  switch  rather  than  one  or  two  two-pole  switches,  on  a 
two-phase  four-wire  circuit. 

.  2.  Single-throw  or  Double-throw. — Single-throw  switches  are 
used,  in  all  ordinary  cases.  A  double-thrown  switch  is  required, 
for  reversing  the  polarity,  for  connecting  a  circuit  to  either  one 
of  two  machines,  and  for  similar  applications  (see  Fig.  55). 


Circuit  1 .-  Circuit  2 

3E 

Outgoing  Feeder, 
Generator  Field, 
Motor,  or  other 
Circuit  Using  or 
Providing  Power 

(a) 

FIG.  55. — Applications  of  double-throw  switches. 

In  (a)  a  two-wire  circuit  may  be  connected  to  either  circuit  1  or  circuit  2.  Three-  and 
four-wire  circuits  may  be  similarly  connected.  If  one  side  of  the  circuit  is  permanently 
closed,  a  single-pole  double-throw  switch  may  be  used. 

In  (6)  a  two-pole  double-throw  switch  is  connected  as  a  reversing  switch.  The  leads  for 
incoming  and  outgoing  power  may  be  i  nterchanged  without  affecting  the  operation  of  the 
switch.  See  Figs.  59  and  61  for  application  of  the  reversing  principle  to  a  control  switch 
and  an  auxiliary  relay,  for  reversing  a  motor. 

In  (c)  the  connections  are  the  same  as  in  (6),  with  the  addition  of  wire  No.  2.  It  is  suit- 
able for  reversing  the  direction  of  rotation  of  induction  motors,  by  reversing  the  sequence 
of  the  phases.  The  same  connection  may  be  used  to  reverse  the  polarity  of  a  D.C.  three- 
wire  system. 

3.  Front-  or  Rear-connected. — Rear-connected  switches  are  used 
on  switchboards;  their  use  in  other  places  has  the  advantage  of 
keeping  the  leads  out  of  the  way,  but  sometimes  the  wiring 
is  made  more  expensive  thereby. 

4.  Rated  Current. — The  current  rating  should  be  such  that  the 
switch  will  carry  that  current  continuously  without  excessive 
heating,  for  as  much  as  an  hour  or  so,  or  until  the  temperature 
becomes  constant. 

5.  Voltage. — The  voltage  to  be  specified  is  the  voltage  between 
poles,  and  unless  otherwise  stated  it  is  the  same  as  the  voltage 
across  the  gap  after  the  circuit  is  opened. 

Special  Applications. — If  any  special  features  are  required,  they 


CONTROLLING  AND  REGULATING  EQUIPMENT    123 


should  be  stated;  e.g.,  if  it  is  used  as  a  quick-break  switch,  or  a 
field  switch  (see  Fig.  42,  page  108,  for  one  form  of  a  field  switch). 
Oil  switches  are  required  if  the  A.C.  voltage  or  current  is  so 
high  that  with  a  knife  switch  there  would  be  excessive  arcing  on 
opening  the  switch.  The  contact  terminals  are  submerged  in 
oil,  so  that  when  the  switch  is  ^__^_ 
opened,  the  oil  flows  into  the 
gap  between  the  terminals, 
insulating  them  from  each 
other  and  stopping  the  cur- 
rent at  the  zero  point.  One 
form  of  oil  switch  is  illus- 
trated in  Fig.  56,  which  shows 
a  single-pole  switch,  or  one 
pole  of  a  two-,  three-,  or 
four-pole  switch1  or  circuit- 


1 

] 

II 

1 

e—  Oil  Level 

:  :: 

i 

G 
^ 

^ 

^ 

-^ 

i 

-•_  • 

7 

— 

j 

FIG.  56. — Arrangement  of  one  type  of 
oil  switch. 


breaker.     Fig.  57  shows  dia- 

grammatically  the  arrangement  of  the  three  poles  in  a  three-pole 

switch.     They  are  all  controlled  by  a  single  handle. 

Disconnecting  switches,  which  are  similar  to  knife  switches, 
are  sometimes  used  on  high  voltages,  to  open  the  circuit  when 
there  is  no  current  flowing  in  the  line.  It  is  often  important  to 
insert  them  on  one  or  both  sides  of  transformers,  circuit-breakers, 

and  other  apparatus,  so  that 
the  apparatus  can  be  insu- 
lated, and  handled  safely  for 
repairs  and  changes.  Fig.  58 
illustrates  a  few  important 
applications  of  disconnecting 


FIG.  57. — Diagrammatic  representation 
of  a  three-pole  oil  switch. 

(6)  As  usually 


(a)  Showing  switch  contacts, 
represented. 


switches. 

Control  switches  are  small 

switches,  ordinarily  of  the 
drum  type.  They  are  us'ed  to  open  and  close  electrically-oper- 
ated switches  and  circuit-breakers  that  are  too  heavy  or  too  far 
away  to  be  controlled  by  hand,  and  for  various  other  purposes. 
Fig.  59  shows  two  such  applications.  A  control  switch  sometimes 
has  a  safety  device,  preventing  the  accidental  opening  and  closing 

1  The  essential  difference  between  an  oil  switch  and  an  oil  circuit-breaker 
is  that  the  breaker  is  made  to  open  on  any  required  overload,  whereas  the 
switch  many  not  be  able  to  stand  such  severe  conditions.  Some  manufac- 
turers call  oil  switches  non-automatic  oil  circuit-breakers. 


124 


ELECTRICAL  EQUIPMENT 


of  circuits;  it  may  also  have  connections  for  indicating  lamps, 
which  show  whether  the  switch  or  circuit-breaker  that  it  controls 
is  open  or  closed.  The  short-time  current  capacity  of  the  control 
switch  must  be  sufficient  to  operate  the  tripping  and  closing 


10 


High  Tension 

Buses 


Outgoing  Feeders 
FIG.  58. — Applications  of  disconnecting  switches. 

1  and  3,  or  2  and  4  for  disconnecting  transformers;  5  for  sectionalizing  buses;  6  and '8,  or 
7  and  9  for  disconnecting  a  circuit-breaker;  10  or  11  for  disconnecting  a  lightning  arrester. 


Auxiliary. 
Circuit 


Oil  Switch  or 
Circuit  Breaker 


Closing  Coil 


Motor  for 
Operating  a 
Field  Rheostat 
or  Controlling 
an  Engine 
Governor 


FIG.  59. — Applications  of  control  switches. 

Each  black  rectangle  represents  a  moving  connector,  mounted  on  the  drum;  each  black 
circle  represents  a  stationary  contact  finger.  When  the  drum  is  turned  to  the  right,  the 
rectangular  connector  makes  connections  between  one  pair  of  contact  fingers;  and  when  to 
the  left,  between  another  pair. 

coils,  or  the  small  motor  used  to  open  and  close  the  circuit- 
breaker.  The  voltage  rating  should  be  the  voltage  of  the  aux- 
iliary circuit  to  which  the  control  switch  is  connected. 

Rheostats  Controlling  Motors  and  Generators. — There  are 
several  different  applications  of  rheostats: 


CONTROLLING  AND  REGULATING  EQUIPMENT    125 

(a)  D.C.  generator  field  rheostats  for  adjustment  of  D.C. 
generator  and  line  voltage. 

(6)  A.C.  generator  field  rheostats  for  adjustment  of  A.C. 
generator  power  factor  and  line  voltage. 

(c)  Exciter  field  rheostats,  which  are  also  for  adjustment  of 
A.C.  generator  and  line  voltage. 

(d)  Shunt-motor   field   rheostats   for    adjustment    of   motor 
speeds. 

(e)  Shunt-motor  armature  rheostats  for  adjustment  of  motor 
speeds. 

(/)  Shunt-motor  starting  rheostats. 

(</)  Series-motor  controllers. 

(ti)  Synchronous-motor  field  rheostats  for  power -factor 
adjustment. 

(i)  Induction-motor  rheostats  for  use  in  series  with  the  motor 
in  starting. 

(j)  Induction-motor  rheostats  similar  to  the  foregoing,  for 
speed  regulation. 

(fc)  Induction-motor  rheostats  for  use  in  series  with  the  line 
and  motor  primary,  for  starting  (an  application  that  is  little 
used) . 

In  prescribing  rheostats  for  the  foregoing,  and  in  general  for 
other  purposes,  it  is  necessary  to  specify: 

1.  The  maximum  current  that  it  is  to  carry. 

2.  Whether  this  current  is  to  be  for  only  a  fraction  of  a  minute, 
or  for  continuous  service  (i.e.,  long  enough  to  reach  practically 
constant  temperature). 

3.  The  total  resistance. 

4.  The  number  of  steps  in  the  resistance. 

5.  The  voltage  for  which  it  is  to  be  insulated  (the  voltage  of 
the  circuit). 

6.  Any  other  information  of  value,  such  as  the  current  when 
all  resistance  is  in  the  circuit,  and  other  details  of  operation. 

The  number  of  steps  should  be  sufficient  for  the  smallest 
necessary  adjustment. 

The  current  capacity  in  the  several  parts  of  the  rheostat  may 
be  graded,  if  desirable,  especially  if  the  current  is  much  less  when 
the  entire  rheostat  is  in  circuit  than  when  it  is  cut  out. 

The  resistance  may  be  embedded  in  enamel,  wound  in  helical 
springs,  wound  on  porcelain,  or  mounted  otherwise.  That  em- 


126  ELECTRICAL  EQUIPMENT 

bedded  in  enamel  is  very  compact,  and  is  satisfactory  if  the  cur- 
rent is  not  in  danger  of  exceeding  the  rating  of  the  rheostat;  but 
an  excessive  current  for  a  short  time  may  be  enough  to  destroy 
the  entire  rheostat,  whereas  in  some  other  forms  the  damaged 
parts  can  be  replaced.  Not  only  the  current-carrying  capacity 
of  the  wire  of  the  rheostat,  but  also  the  ventilation  or  other 
means  of  cooling  must  have  careful  attention,  especially  in  a 
rheostat  absorbing  a  large  amount  of  power  for  a  long  period  of 
time. 

In  case  a  rheostat  is  at  a  distance  from  the  switchboard,  it 
can  be  operated  by  means  of  a  small  motor,  controlled  by  a  con- 
trol switch,  as  illustrated  in  Fig.  59. 

AUTOMATIC  REGULATING  EQUIPMENT 

This  equipment  is  employed  for  maintaining  constant  current 
and  constant  voltage,  and  occasionally  for  some  other  purposes. 
Constant-current  and  constant-voltage  regulating  transformers 
are  explained  in  Chapter  XIV.  We  shall  consider  some  further 
voltage-regulating  equipment. 

The  importance  of  voltage  regulation  can  hardly  be  overesti- 
mated, where  power  is  used  for  lighting,  or  for  testing  purposes. 
At  best  it  is  not  commercially  practicable  to  have  the  voltage 
strictly  constant,  but  voltage-regulating  equipment  can  be  pro- 
vided that  holds  the  voltage  within  a  fraction  of  1  per  cent,  of  a 
constant  value. 

Generator  Voltage  Regulator. — The  elements  of  this  rather 
complicated  device  are  illustrated  in  Fig.  60.  Ra,  Ra  are  alter- 
nator field  rheostats,  and  Re  the  exciter  field  rheostat.  When  the 
bus  voltage  is  too  high,  the  plunger  in  the  voltage  regulator  is 
drawn  up,  and  the  contact  is  opened.  This  puts  the  resistance 
of  the  exciter  rheostat  in  series  with  the  exciter  field,  the  exciter 
voltage  then  drops,  decreasing  the  alternator  field  current,  and 
so  the  voltage  of  the  alternator.  As  soon  as  the  voltage  is  too 
low,  the  plunger  in  the  voltage  regulator  drops,  the  contact 
closes,  and  the  voltage  is  raised.  This  regulation  takes  place 
very  rapidly,  and  is  a  most  satisfactory  method  of  maintaining 
a  constant  bus  voltage.  On  account  of  the  rapid  operation  of 
the  regulator,  the  per  cent,  variation  of  voltage  is  very  slight. 

This  type  of  regulator  renders  its  greatest  service  when  applied 
to  A.C.  generators,  because  their  voltage  regulation  is  so  much 


CONTROLLING  AND  REGULATING  EQUIPMENT    127 

poorer  than  that  of  compound  D.C.  generators.  But  the  regula- 
tion of  even  D.C.  generators  is  improved  by  such  a  voltage 
regulator.  If  the  D.C.  generator  is  not  too  large,  the  regulator 
may  be  connected  to  act  directly  on  the  generator  field  current; 


Voltage 

Eegulator 


"V/wJ  Voltage  Transformer 

(it  used)  A.C.Buses 


Exciter  Alternators' 

FIG.  60. — Generator  voltage  regulator  and  connections. 

In  many  cases  the  bus  voltage  is  too  high  to  connect  directly  to  the  voltage  regulator. 
A  voltage  transformer  is  then  employed  to  step  the  voltage  down  to  a  suitable  value. 


but  for  very  large  generators,  it  is  well  to  employ  exciters  very 
much  as  they  are  employed  for  alternators. 

A  voltage -regulating  relay,  or  contact-making  voltmeter,  is 
an  instrument  somewhat  like  the  voltage  regulator  just  described, 
but  its  operation  is  altogether  different;  it  regulates  the  voltage 


Voltage 
D.C.Auxiliary  Buses         Belay 


oltage 
ransformer 

(it  used) 
Motor/-~n 


A.C.Busea 


3-Pbase 
Induction 
Feeder  Voltage 

Eegulator 


Outgoing 
Feeder 


FIG.  61. — Voltage  regulating  relay  and  auxiliary  relay,  regulating  the 
feeder  voltage  by  means  of  an  induction  regulator.  (The  internal  connec- 
tions of  the  induction  regulator  are  illustrated  in  Fig.  45,  p.  112.) 


of  a  feeder,  instead  of  that  of  a  generator.  The  connections  of 
the  voltage-regulating  relay  are  shown  in  Fig.  61.  The  actual 
raising  of  the  voltage  is  done  by  the  three-phase  induction  regu- 
lator ,Aas  explained  in  Chapter  XIV.  A  small  motor  is  connected 


128  ELECTRICAL  EQUIPMENT 

by  gear  and  worm  wheel  to  rotate  the  induction  regulator,  and  it 
is  the  province  of  the  relay  to  run  the  motor  as  required,  to  main- 
tain constant  voltage.  The  winding  of  the  relay  is  connected 
to  the  secondary  of  a  voltage  transformer  whose  primary  is 
across  one  phase  of  the  feeder.  When  the  feeder  voltage  is  too 
high,  the  plunger  of  the  relay  rises,  operates  a  contact  lever,  and 
closes  the  contact  L;  and  when  the  voltage  is  too  low,  it  closes 
contact  H.  The  leads  from  the  relay  contacts,  H  and  L,  are 
connected  so  as  to  operate  the  motor.  On  account  of  the  deli- 
cate construction  of  the  relay,  its  contacts  are  not  heavy  enough 
to  carry  the  motor  current,  and  an  additional  relay,  called  a 
relay  switch  or  auxiliary  relay,  is  so  arranged  that  when  the  con- 
tact of  the  regulating  relay  is  closed  a  small  current  passes  through 
the  coil  of  the  auxiliary  relay,  which  has  contacts  of  heavier 
construction,  capable  of  carrying  the  full  motor  current.1  When 
these  contacts  are  closed,  the  motor  is  rotated  in  the  required 
direction  until  the  voltage  becomes  normal.  The  diagram  shows 
connections  from  D.C.  auxiliary  buses,  operating  a  D.C.  motor. 
Sometimes  A.C.  auxiliary  buses,  and  an  induction  motor,  are 
employed. 

It  is  a  matter  of  great  industrial  importance,  that  by  means 
of  this  equipment  several  feeders  of  different  lengths  can  be  taken 
from  a  single  set  of  buses,  and  even  though  the  load  fluctuations 
on  each  feeder  are  independent  of  those  on  any  other,  the  voltage 
may  be  held  practically  constant  at  the  beginning  or  end,  or  at 
any  other  point  on  each  feeder.  If  it  is  to  be  kept  constant  at 
any  point  except  the  beginning,  it  is  possible  to  compensate  for 
line  drop,  as  explained  in  the  next  paragraph. 

Line-drop  Compensator. — Thus  far  we  have  considered  only 
such  voltage-regulating  equipment  as  would  maintain  constant 
voltage  at  the  buses.  The  results  would  be  like  those  with  a 
D.C.  flat-compound  generator,  in  that  no  provision  has  been 
made  for  line  drop.  The  voltage  at  the  end  of  the  line  is  less 
than  at  the  buses,  on  account  of  both  resistance  and  reactance 
of  the  line.  The  essentials  of  a  simple  and  effective  means  of 
compensating  for  voltage  drop  are  shown  in  Fig.  62.  The 
primary  of  a  voltage  transformer  is  connected  between  buses  C 
and  A.  This  gives  a  secondary  voltage  corresponding  to  the 

1  This  use  of  an  auxiliary  relay,  to  utilize  a  small  current  in  performing 
a  heavy  operation  is  not  uncommon,  especially  in  connection  with  relays 
that  are  delicately  constructed  for  very  sensitive  operation. 


CONTROLLING  AND  REGULATING  EQUIPMENT    129 

bus  voltage;  the  compensator  is  used  where  it  is  desirable  to 
find  how  the  line  drop  affects  the  voltage.  A  resistance,  R, 
and  reactance,  X,  are  adjusted  so  that  when  they  are  connected 
in  the  current  transformer  secondary  circuit,  their  effects  are 
equivalent  to  line  resistance  and  reactance.  That  is,  if  a  current 
in  one  of  the  main  conductors  is  such  as  to  produce  a  drop  that 
is  10  per  cent,  of  the  bus  voltage,  the  secondary  current  produces 
a  drop  that  is  10  per  cent,  of  the  secondary  of  the  voltage  trans- 
former. Two  current  transformers  are  shown  in  the  diagram — 
one  on  line  C  and  one  on  line  A — because  in  correcting  the  CA 


A.  0.  Buses 


FIG.  62. — Line-drop  compensator  and  connections. 

voltage  we  must  take  account  of  the  drop  produced  in  the  two 
lines,  by  their  respective  currents.  In  actual  construction  the 
compensator  is  modified  slightly  from  that  here  shown,  but  the 
principle  of  operation  is  as  indicated.  Some  line-drop  compen- 
sators are  made  to  compensate  for  any  resistance  drop  not  ex- 
ceeding 6  per  cent,  (at  full  load  current),  and  any  reactance 
drop  not  exceeding  6  per  cent.  Others  compensate  for  larger 
resistance  and  reactance  drops — sometimes  as  much  as  36  per 
cent.  each. 

A  voltmeter  connected  to  the  voltage  transformer  and  line- 
drop  compensator,  as  in  Fig.  62,  indicates  the  voltage  at  the  end 
of  the  line,  even  though  the  equipment  is  at  the  power  station. 
A  voltage-regulating  relay  with  these  connections  maintains 
a  constant  voltage  at  the  terminus,  just  as  it  would  maintain 
a  constant  voltage  at  the  power  station  if  the  line-drop 
compensator  were  emitted. 


CHAPTER  XVII 
CIRCUIT-BREAKING  EQUIPMENT1 

Protection  of  electric  apparatus  and  circuits  is  equivalent  to 
insurance.  The  greater  the  possibility  of  trouble,  and  the  greater 
the  loss  in  case  of  trouble,  the  more  important  it  is  to  guard 
against  it.  Automatic  protection  falls  into  two  classes:  Protec- 
tion by  circuit-breaking  equipment  against  overload,  short- 
circuit  and  other  dangerous  conditions  of  the  circuit;  and  pro- 
tection by  lightning-arrester  equipment  against  sudden  and 
excessively  high  voltages,  due  to  lightning  and  other  line  disturb- 
ances. Circuit-breaking  equipment  is  treated  in  this  chapter, 
and  lightning-arrester  in  Chapter  XVIII. 

Fuses,  carbon  circuit-breakers,  oil  circuit-breakers,  and  relays 
comprise  the  essentials  of  circuit-breaking  equipment. 

FUSES 2 

Fuses  are  more  commonly  used  on  low  than  on  high  voltage, 
and  for  small  than  for  large  currents;  although  they  are  used  in 
some  cases  on  all  voltages  up  to  110,000,  and  for  currents  (at  low 
voltages)  as  high  as  1200  amp.  They  are  not  applicable  to  cir- 
cuits where  the  total  kilowatt  capacity  of  the  generators  back 
of  the  fuses  is  very  large,  in  which  case  circuit-breakers  should  be 
used;  nor  are  they  applicable  to  circuits  requiring  selective  open- 
ing.3 Fuses  are  particularly  applicable  to  low- voltage  lighting 
circuits  and  the  primaries  of  transformers,  and  to  similar  cases . 
where  a  relatively  small,  inexpensive  piece  of  apparatus  is  re- 
quired, which  does  not  operate  often,  but  which  offers  positive 
protection  against  a  short-circuit.  Obviously  fuses  are  not 
applicable  to  cases  where  they  would  be  blown  out  frequently, 
for  the  cost  of  fuses  would  be  unnecessarily  large. 

The  fusing  material  is  sometimes  German  silver  or  aluminum, 

1  See  references  at  the  beginning  of  Chapter  XVI,  p.  121. 

2  See  S.  13: 103, 104,  General  Principles,  Comparison  with  Circuit-breakers. 

3  That  is,  opening  only  in  certain  cases  of  overload;  this  is  explained  in 
the  discussion  of  Relays,  p.  140. 

130 


CIRCUIT-BREAKING  EQUIPMENT  131 

but  more  often  an  alloy  with  a  low  melting  temperature.  Usually 
fuses  are  enclosed,  to  prevent  the  melted  metal  from  flying  and 
doing  damage,  and  also  in  some  cases  to  quench  the  arc.  There 
are  three  forms  of  enclosed  fuses:  the  Edison  plug-type  fuses, 
which  fit  the  thread  of  an  ordinary  Edison  lamp  socket,  and  are 
suitable  for  small  currents  and  low  voltages;  cartridge  fuses  with 
ferule  contacts,  which  are  suitable  only  for  small  currents  but 
can  be  used  on  higher  voltages;  and  cartridge  fuses  with  knife- 
blade  contacts,  for  large  currents  and  higher  voltages.1 

For  a  large  current  at  a  high  voltage,  sometimes  an  ''expulsion 
fuse"  is  employed — that  is,  a  fuse  mounted  in  a  holder  shaped 
like  a  gun,  that  blows  out  the  vaporized  fuse  by  the  force  of  its 
own  expansion. 

CIRCUIT-BREAKERS 

A  circuit-breaker  must  be  provided  of  such  size  and  construc- 
tion that  there  is  no  excessive  heating  at  the  contacts  or  in  the 
conductors,  when  it  is  carrying  the  required  current.  The  space 
between  all  parts  that  are  alive  when  the  breaker  is  open  must  be 
sufficient  to  prevent  arcing  from  one  terminal  to  another;  such 
arcing  is  liable  to  occur  at  the  time  of  opening  the  breaker, 
especially  under  conditions  of  a  bad  short-circuit.  Based  on 
these  two  requirements,  a  circuit-breaker  must  have  a  sufficient 
" rated  ampere  capacity"  and  a  sufficient  " ultimate  breaking 
capacity,"  as  follows: 

The  rated  ampere  capacity  should  be  based  on  the  maximum 
current  that  the  line  is  expected  to  carry.  For  example,  if 
there  is  one  motor  on  the  line,  whose  normal  current  is  100  amp., 
but  which  may  be  expected  to  carry  a  25  per  cent,  overload,  a 
125-amp.  breaker  is  required.  If  the  overload  is  for  less  than  an 
hour,  the  additional  heating  of  the  circuit-breaker  is  small 
enough  so  that  in  some  cases  it  may  be  disregarded. 

The  following  carbon  breakers  are  obtainable:  1,  2,  3,  and  4 
pole;  250,  300,  600,  750  volts;  5,  10,  25,  50,  75,  100,  150,  200, 
300,  400,  600,  and  800  amp. 

The  ultimate  breaking  capacity,  expressed  in  amperes,  is 
the  largest  current  that  the  breaker  is  expected  to  open.  This 
may  be  many  times  the  rated  full-load  current  of  the  circuit. 
Before  deciding  what  this  must  be,  several  facts  should  be  noted : 

1  See  National  Electrical  Code  for  allowable  ratings. 


132  ELECTRICAL  EQUIPMENT 

1.  The  best  engineering  practice  is  to  put  circuit-breakers  in 
D.C.  generator  and  feeder  circuits,  and  in  A.C.  feeder  circuits,  but 
not  in  A.C.  generator  circuits. 

2.  A  D.C.  generator  may  deliver,  on  short-circuit,  for  an  in- 
stant, about  30  times  full-load  current,  but  the  heavy  current 
rapidly  demagnetizes  the  field,  reducing  the  voltage  and  the  short- 
circuit  current,  so  that  after  2  sec.,  the  breaker  need  only  be  one 
that  will  open  10  times  full-load  current  at  rated  voltage. 

3.  An  A.C.  generator  will  deliver  on  short-circuit,  for  an  in- 
stant, usually,  12  times  full-load  current,  but  the  heavy  current 
demagnetizes  the  field,  so  that  after  2  sec.  the  breaker  need  only 
be  one  that  will  open  2  to  3  times  full-load  current  at  rated  voltage. 

4.  If  several  A.C.  generators  are  connected  in  parallel  to  the 
same  buses,  the  short-circuit  current  in  a  feeder  is  the  sum  of  the 
short-circuit  currents  of  all  the  generators,  because  the  A.C. 
generators  themselves  have  individually  no  overload  protection. 

5.  If  the  circuit-breaker  is  in  the  secondary  circuit  of  a  trans- 
former, or  at  a  distance  out  on  a  line,  the  short-circuit  current 
is  reduced  by  transformer  or  line  impedance.     If  the  transformer 
capacity  is  small  compared  with  that  of  the  generator,  the  short- 
circuit  current  depends  chiefly  on  the  transformer,  rather  than 
on  the  generator.     Ordinary    transformers    with  high-tension 
windings  of  16,500  volts  or  less  have  an  impedance  drop  of  2.5 
to  4  per  cent,  at  full-load,  so  that  the  short-circuit  current  (i.e.,  at 
100  per  cent,  drop)  is  from  40  to  25  times  full-load  current. 

Based  on  these  facts,  the  size  of  the  breaker  is  different  under 
different  conditions  of  service.  The  following  figures  are  typical, 
applying  approximately  in  ordinary  cases: 

D.C.  Generator  Circuit. — If  the  breaker  is  set  for  instantaneous 
operation,  it  should  have  an  ultimate  capacity  of  30  times  full- 
load.  If  the  operation  of  the  breaker  is  delayed  for  2  sec.1  the 
ultimate  capacity  need  be  only  10  times  full-load. 

D.C.  Feeder  Circuit. — Breaker  ultimate  capacity  should  be  10 
to  30  times  the  sum  of  the  full-load  currents  of  all  the  generators. 

For  example,  if  four  D.C.  generators  are  in  operation  at  one  time,  and 
each  generator  capacity  is  100  amp.,  the  ultimate  breaking  capacity  of 
each  feeder  breaker  should  be  4,000  to  12,000  amp.,  even  though  it  may 
carry  normally,  say,  100  amp. 

A.C.  Feeder  Circuit. — If  a  feeder  circuit-breaker  is  in  the  sec- 
ondary circuit  of  a  transformer,  the  ultimate  breaking  capacity 
1  By  a  method  explained  later  in  the  chapter. 


CIRCUIT-BREAKING  EQUIPMENT  133 

need  not  exceed  25  to  40  times  the  transformer  rated  capacity. 
(Information  can  be  obtained  from  the  manufacturer,  as  to  the 
short-circuit  current  on  any  transformer.) 

If  the  breaker  is  at  a  distance  from  the  generator,  the  ultimate 
breaking  capacity  need  not  exceed  the  short-circuit  current  on 
the  line,  which  is  the  rated  line  voltage  divided  by  the  line 
impedance. 

For  example,  on  a  2,200-volt,  60-cycle  circuit,  400  ft.  long,  consisting  of 
No.  1  stranded  conductors,  spaced  15  in.  apart  (see  Table  XII,  page  81): 
Line  resistance  for  two  wires     =  0.1288  X  2  X  400/1,000  =  0.103  ohm. 
Line  reactance  for  two  wires     =  (0.0491  +  0.0621)  X  2  X  400/1,000  = 

0.0890   ohm. 

Line  impedance  for  two  wires    =  V0.1032  +  0.08902          =  0.136  ohm. 
Short-circuit  current  =2,200/0.136  =  16,200  amp., 

which  is  the  required  ultimate  capacity  of  the  breaker. 

If  the  breaker  is  near  the  generator,  is  not  in  the  secondary 
circuit  of  a  transformer,  and  opens  instantaneously,  its  ultimate 
breaking  capacity  should  be  about  12  times  the  rating  of  the 
generator.  If  there  are  two  or  more  generators,  it  should  be 
12  times  the  sum  of  the  rated  capacities.  If  the  breaker  does 
not  open  for  2  sec.,  its  ultimate  capacity  need  be  only  2  or  3  times 
the  sum  of  the  generator  capacities,  and  if  more  than  2  sec. 
the  capacity  is  still  more  reduced. 

Thus  if  there  are  three  1,000-kva.,  2,200-volt,  three-phase  alternators  in 
a  station,  and  the  circuit-breakers  are  set  for  a  2-sec.  time  limit,  the  ultimate 
breaking  capacity  of  the  circuit-breakers  should  be  3  X  3  X  1,000  X 
1,000/(2,200  X  1.73)  or  2,370  amp. 

From  these  computations  it  is  possible  to  find  the  approximate 
size  of  circuit-breaker;  but  as  the  manufacturer  knows  the  limi- 
tations of  each  breaker,  he  will  prefer  to  specify  the  size  of 
breaker  to  use,  basing  his  choice  on  the  following : 

1.  The  maximum  continuous  load. 

2.  The  maximum  possible  instantaneous  overload. 

3.  The  maximum  overload  that  all  the  generators  can  sustain 
(for  a  fraction  of  a  minute)  on  short-circuit. 

4.  The  maximum  time  element  in  the  operation  of  the  breaker, 
that  can  be  allowed  with  safety  to  the  system. 

5.  Length,  size  and  spacing  of  conductors  from  the  generator 
to  the  circuit-breaker,  and  frequency  of  the  system. 

6.  The  size  of  transformer,  if  the  breaker  is  in  a  transformer 
secondary. 


134  ELECTRICAL  EQ  UIPMENT 

Oil  circuit-breakers  are  required  where  the  current  or  voltage 
is  so  high  that  a  carbon  breaker  is  not  sure  to  open  the  circuit  on 
overload  or  short-circuit.  They  are  nearly  always  used  on 
over  750  volts,  and  in  many  cases,  especially  on  A.C.  circuits, 
they  are  used  on  lower  voltages. 

The  contacts  are  submerged  in  oil,  and  in  general  the  con- 
struction of  an  oil  circuit-breaker  is  similar  to  that  of  an  oil 
switch,  such  as  is  described  and  illustrated  in  Chapter  XVI, 
page  123. 

When  the  breaker  opens  under  the  worst  conditions,  an  arc 
forms  across  the  gap.  This  arc  is  vaporized  metal  and  oil,  which 
therefore  expands  and  puts  the  oil  under  considerable  pressure. 
The  design  of  the  breaker  must  be  adequate  to  stand  the  pres- 
sure, and  to  prevent  the  oil  from  rushing  out  under  the  heavy 
pressure  through  the  opening  in  the  top  of  the  tank;  the  size  of 
the  contacts  must  be  sufficient  to  prevent  being  burned  up 
after  a  few  operations;  and  the  insulating  distance  through  oil 
to  other  parts  of  the  Breaker  must  be  enough  so  that  an  arc 
will  not  be  established  during  the  commotion  of  opening  on  short- 
circuit.  The  higher  the  voltage,  and  the  larger 
the  current,  the  larger  the  tanks  must  be. 

I  For  three-phase  circuits,  the  three  lines  are 

(5)  a  (|> --Trip-con     opened  by  a  single  breaker.     Sometimes  the 

three  poles  of  the  breaker  are  in  separate  oil 

L        6  3  _o . ,      tanks,  but  the  operating  mechanism  is  all  in 

circuit-breaker,      one  system,  so  that  there  is  essentially  only 

operated    by   two      one  operation  in  opening  all  the  poles.     Two- 

tnp-coils.  r  •  • 

phase  tour-wire   and  three-wire   circuits   re- 
quire respectively  four-pole  and  three-pole  breakers. 

An  oil  circuit-breaker  is  tripped  either  by  a  trip-coil  in  series 
with  the  line,  or  by  a  coil  connected  to  some  other  circuit.  Fig. 
63  is  a  diagram  representing  a  three-pole  oil  breaker  that  is 
tripped  by  coils  in  series  with  two  of  the  conductors  of  the  three- 
phase  line.  As  an  overload  or  short-circuit  cannot  occur  under 
ordinary  conditions  in  the  third  conductor,  without  flowing  also 
in  one  or  both  of  the  other  two,  it  is  frequently  considered  ade- 
quate to  provide  only  the  two-pole  protection.  But  there  is  a 
possibility  of  circumstances  in  which  both  the  generator  and  the 
third  conductor  become  grounded,  as  in  Fig.  64.  The  current 
then  flows  from  the  generator  through  the  ground  to  the  grounded 
wire  and  back,  without  flowing  through  either  trip-coil.  For 


CIRCUIT-BREAKING  EQUIPMENT 


135 


this  reason,  as  an  added  precaution,  sometimes  the  third  pole 
of  the  breaker  also  has  overload  protection. 

Trip-coils  in  series  with  the  main  conductors  are  satisfactory, 
if  there  are  no  restrictions  as  to  the  time  and  circumstances  of 
operation  of  the  breaker;  but 
the  difficulties  already  men- 
tioned, and  others  make  it  im- 
portant in  many  cases  that  the 
breaker  exercise  a  high  degree  of 
discretion  in  its  operation.  The 
breaker  may  then  be  tripped 
from  an  auxiliary  circuit,  as  in 
Fig.  65,  by  a  trip-coil  that  oper- 
ates when  a  contact,  R,  is  closed 
by  a  relay.1  If  the  breaker  is  FIG.  64.— A  possible,  but  improba- 
at  a  distance  from  the  switch-  J>le  condition  of  grounding,  in  which 
.  .  two  trip-coils  are  inadequate  for 

board  where  it  is  controlled,  or    protecting  the  circuit. 

if  it  is  too  heavy  to  be  con- 
trolled by  hand,  a  coil  is  provided  for  closing,  as  well  as  for 
opening  the  breaker.  Such  a  closing  coil  is  shown  in  Fig.  65, 
which  is  operated  by  closing  contact  C  by 
hand.  For  tripping  (opening)  the  breaker  by 
hand,  contact  T  is  employed. 

Carbon  circuit-breakers  are  ordinarily 
limited  to  A.C.  and  D.C.  circuits  of  750 
volts  or  less.  The  operation  of  the  breaker 
in  its  simplest  form  is  as  follows:  when  an 
overload  occurs,  an  electromagnetic  device 

FlQ   65 Elec-    releases  the  arm  of  the  breaker,  which  flies 

trically-operated  cir-    open.     As  it  starts  to  open,  it  separates  the 

C™-c7i!tabtr'    closed    mam  c°PPer  contacts,  and  the  current  is  de- 
automaticaiiy  on  over-    fleeted  to  flow  through  a  carbon  contact.     As 

load,  to  trip  the  breaker.  & 

r= contact      closed    the  breaker  opens  still  further,   the  carbon 

manually    to    trip    the  r 

breaker.  contacts  open;  by  this  time  the  copper  con- 

C  =  Contact        closed  J 

manually  to  open  the    tacts  have  moved  away,  so  that  there  is  no 

breaker. 

arcing  of  the  copper,  but  it  is  restricted  to 
the  carbon.  The  non-arcing  tendency  of  the  carbon  allows  the 
breaker  to  open  without  excessive  arcing.  In  some  breakers  there 
is  an  intermediate  contact  of  phosphor-bronze,  that  breaks  after 
the  copper,  but  before  the  carbon  contacts.  This  serves  to 
1  See  Relays,  pp.  136-141. 


136  ELECTRICAL  EQUIPMENT 

protect  the  copper  in  case  the  carbon  contacts  are  burned  away, 
or  for  any  reason  fail  to  operate. 

Some  two-pole  carbon  breakers  are  arranged  so  that  one  pole 
may  be  closed  at  a  time,  but  both  poles  are  opened  automatic- 
ally by  an  overload.  The  advantage  of  this  arrangement  is  that 
it  is  not  necessary  to  provide  a  knife  switch  to  use  along  with  the 
breaker. 

The  electromagnetic  device  that  trips  the  breaker  is  usually 
energized  by  the  main  current  of  the  breaker,  but  instead  of  this 
or  in  addition  to  it  the  breaker  may  be  actuated  by  a  device 
operating  on  underload  (when  the  current  becomes  less  than  a 
prescribed  amount),  or  undervoltage.  An  example  of  underload 
operation  is  in  case  of  charging  a  storage  battery.  When  the 
battery  becomes  charged,  the  current  drops  off  and  the  breaker  is 
opened.1  An  example  of  undervoltage  operation  is  the  discon- 
necting of  a  motor,  if  for  any  reason  the  line  is  temporarily 
dead.  When  the  power  comes  on  the  line  again,  if  the  motor 
were  not  disconnected,  it  might  be  damaged  by 
the  excessive  current  that  would  flow. 

Sometimes   the   breaker   is   provided   with   a 
trip-coil  that  is  operated  by  some  outside  appa- 
ratus, as  in  Fig.  66.     The  coil  has  no  current  in 
it,  until  the  contact,  R,  is  closed.     The  contact, 
R,  may  be  closed  by  hand,  or  in  case  of  any  ab- 
R  -     normal  condition  of  the  circuit  it  may  be  closed 
Carbon  circuit     automatically.2 

breaker,  tripped  Carbon  breakers  can  be  obtained  with  one, 
is7  energized  by  two>  three  or  four  poles  as  required.  As  they 
closing  a  con-  are  used  more  often  on  D.C.  than  on  A.C.  cir- 
cuits, the  breakers  are  in  most  cases  single-pole 
or  two-pole.  The  size  of  breaker  must  be  sufficient  to  carry  the 
rated  current  and  to  open  any  possible  overload  or  short-circuit, 
as  already  explained. 

PROTECTIVE  RELAYS 

Relays  are  either  protective  or  regulating.  Examples  of 
regulating  relays  were  explained  in  Chapter  XVI.  In  this  chap- 
ter only  protective  relays  are  considered. 

1  However,  this  is  not  the  best  criterion  on  which  to  limit  the  battery 
charge. 

8  See  Relays,  p.  139,  Fig.  70. 


Circuit- 
Breaker 


CIRCUIT-BREAKING  EQUIPMENT 


137 


12 


10 


A  protective  relay  is  used  to  operate  a  circuit-breaker,  where 
it  is  necessary  to  exercise  much  discretion  in  opening  the  circuit. 
If  instantaneous  operation  is  permissible  on  overload,  the  breaker 
can  be  set  to  operate  satisfactorily  without  a  relay;  and  it  is  even 
possible  to  make  the  breaker  delay  its  operation,  thereby  introduc- 
ing a  time  element.  But  the  most  accurate  control  of  the  time 
element,  and  various  other  desirable  features  are  obtained  only 
by  means  of  relays. 

A  protective  relay  is  an  electromagnetic  device  that  opens  or 
closes  a  contact  and  thereby  operates  a  circuit-breaker,  when 
certain  abnormal  conditions  exist  on  the  line  that  is  being  pro- 
tected. Thus  relays  are  employed  to  open  the  breaker  in  case  of 
overload,  underload,  overvoltage, 
undervoltage,  overspeed,  reversal  of 
flow  of  power,  and  various  other 
abnormal  conditions.  Of  these, 
overload  relays  are  in  more  general 
use  than  any  of  the  others.  Relays 
are  somewhat  like  meters  in  their 
operation,  except  that  they  are  more 
rugged;  and  whatever  can  be  meas- 
ured by  any  electric  meter  can  be 
used  to  operate  a  relay,  and  thereby 
a  cicuit-breaker. 

Time  Limit. — Just  as  the  motion 
of  a  meter  can  be  damped,  so  the 
operation  of  a  relay  may  be  made 
slow.  In  some  relays  the  time  of 
operation  is  the  same  under  all  con- 
ditions of  overload,  and  in  others  it 
is  less  in  case  of  a  heavy  overload 
than  when  the  overload  is  slight. 
The  first  is  known  as  a  definite  time- 
limit,  and  the  second  as  an  inverse  time-limit  relay.  The  name 
does  not  mean  that  the  time  is  strictly  in  inverse  proportion  to 
the  load,  but  that  in  general  at  larger  overloads  the  time  is  less. 
This  inverse  time-limit  is  of  great  importance,  because  the  cir- 
cuit is  not  opened  without  giving  an  opportunity  for  the  overload 
to  stop;  and  the  less  the  overload  the  longer  is  it  safe  to  leave  the 
circuit  closed. 

In  Fig.  67  are  curves  showing  the  operation  of  an  inverse  time- 


ice          300         500 

Percent  of  Maximum  Continuous  Load 

FIG.  67. — Time-load  curves  of 
an  inverse  time-limit  relay. 


138 


ELECTRICAL  EQUIPMENT 


limit  relay.  The  lower  curve,  representing  the  operating  charac- 
teristics with  setting  A,  shows  that  if  the  current  is  125  per  cent, 
of  the  maximum  continuous  load  (25  per  cent,  overload),  the 
relay  operates  in  about  3  sec.;  but  at  400  per  cent.,  it  operates  in 
0.3  sec.  With  settings  B  and  C  the  time  is  longer  in  each  case; 
but  with  any  setting  the  time  is  relatively  long  at  small  over- 
loads, and  very  short  on  heavy  overloads.  If  the  load  is  heavy 
enough,  the  time  with  any  setting  is  less  than  2  sec.  It  will  be 
remembered,  however,  that  the  circuit-breakers  have  less  work 
to  perform  if  there  is  a  time  element  of  at  least  2  sec. 

The  operation  of  a  definite  time-limit  relay  is  very  different  from 
Fig.  67.  The  relay  can  be  set  to  operate  in  the  required  number 
of  seconds,  and  it  will  take  that  length  of  time,  whatever  the  over- 
load. This  type  of  relay  obviously  has  the  advantage  of  not 
tripping  the  circuit-breaker  too  quickly  on  heavy  overloads. 

Fig.  68  shows  the  operation  of  a  relay  that  combines  the  definite 
and  inverse  time  limits.  The  time  is  long  on  a  slight  overload, 


CHARACTERISTIC  TIME  CURVE 

N'lOTIMESniWG 
Time  is  proportional  to  lever  setting 


%  OF  AMPERES  NECESSARY  TO  CLOSE  CONTACTS 


2000 

FIG.  68. — Time-load  curve  of  an  inverse  time-limit  relay  having  a  definite 

minimum  time. 

and  shorter  on  a  heavy  overload,  but  it  is  never  appreciably  less 
than  a  certain  minimum  which  can  be  made  large  or  small,  as 
desired.  With  No.  10  setting,  for  example,  at  200  per  cent,  of 
maximum  continuous  load  (100  per  cent,  overload),  the  relay 
operates  in  5  sec.;  at  1,000  per  cent,  it  operates  in  only  2  sec.; 
but  at  heavier  overloads  there  is  hardly  any  appreciable  decrease 
in  time.  This  operation  is  not  so  severe  on  the  circuit-breaker 
as  the  ordinary  inverse  time  limit;  and  the  system  is  not  as  likely 
to  be  tied  up  unnecessarily  by  a  slight  overload,  as  if  the  relays 
had  a  definite  time  limit.  There  is  another  important  use  of  this 
relay,  in  case  two  breakers  are  in  series.  Let  us  consider  breaker 
A ,  Fig.  69,  which  is  on  a  feeder  circuit,  and  is  located  at  the  power 


CIRCUIT-BREAKING  EQUIPMENT 


139 


station;  and  breaker  B,  which  is  on  a  motor  circuit  branching 
from  the  feeder.  If  the  relay  at  the  motor  has  a  1-sec.  setting, 
and  the  one  at  the  power  station  a  2-sec.  setting,  as  in  Fig.  68, 
there  is  no  possibility  that  motor  trouble  will  tie  up  the  entire 
feeder,  because  the  motor  breaker  will  open  ahead  of  the  feeder 
breaker,  and  restrict  the  trouble  to  the  motor  circuit.  But  if 
the  relay  of  Fig.  67  is  used  on  both  breakers,  the  difference  in 
time  between  settings  A  and  B  is  so  little  that  both  breakers  may 
be  opened,  and  the  entire  feeder  will  be  temporarily  tied  up. 


To 
Motor 


FIG.  69. — Circuit-breakers  which 
should  preferably  be  operated  by  re- 
lays having  inverse  time  limit  with 
definite  minimum. 


Auxiliary 
Circuit 

FIG.  70. — Two  overload  relays 
and  two  current  transformers  pro- 
tecting a  three-phase  circuit. 


Applications. — Relays  used  on  A.C.  circuits  are  not  connected 
in  series  with  the  line,  but  are  used  with  current  transformers.1 
The  connection  of  two  relays  to  two  current  transformers  on  a  three- 
phase  circuit  is  illustrated  in  Fig.  70.  If  there  is  an  overload 
on  either  outside  conductor,  at  least  one  of  the  relays  closes  its 
contact,  and  the  breaker  is  opened  by  a  current  flowing  from  the 
auxiliary  circuit,  through  the  contact  of  that  relay,  and  through 
the  trip-coil.  The  circuit  is  fully  protected  by  this  arrangement 
except  in  a  special  case  of  grounding,  such  as  is  illustrated  in 
Fig.  64,  page  135. 

Fig.  71  is  the  same  as  Fig.  70,  except  that  there  are  three 
instead  of  two  current  transformers,  offering  the  full  protection 
on  three  conductors  which  is  not  afforded  by  Fig.  70.  There 
are  only  two  relays,  but  each  takes  the  resultant  of  the  secondary 

1  See  Chapter  XV. 


140 


ELECTRICAL  EQUIPMENT 


currents  from  two  of  the  current  transformers.  If  it  is  remem- 
bered that  no  current  can  flow  in  the  transformer  secondary, 
without  a  corresponding  primary  current,  it  can  be  shown  that 
any  possible  overload,  short-circuit  or  ground  will  operate  one 
or  both  of  the  relays,  and  so  the  circuit-breaker.  Three  relays 
are  sometimes  used  in  this  case  instead  of  two,  but  the  added 
relay  is  superfluous.  This  combination  of  three  current 
transformers  and  two  relays  is  known  as  the  Z-connection. 

By  an  ingenious  arrangement  of  connections,  it  is  possible  to 
utilize  the  current  from  the  transformers,  to  trip  the  breaker 


*  \  Current 
Transformers 

J— crrl— I   111' 

Overload 
Belays 


Auxiliary 
Circuit 

FIG.  71. — Two  overload  relays  and  three  "Z-connected"  current  trans- 
formers protecting  a  three-phase  circuit. 

without  the  necessity  of  an  auxiliary  circuit,  but  the  standard 
connections  of  Figs.  70  and  71  are  sufficient  to  illustrate  the 
applications. 

Another  application  of  relays  is  on  parallel  feeders.  In  Fig. 
72,  there  are  two  three-phase  lines  feeding  from  the  generating 
station  to  the  substation.  This  is  done,  so  that  if  there  is  a 
break  or  short-circuit  in  one  line  the  other  will  carry  the  power. 
But  if  a  short-circuit  occurs  at  S,  the  current  on  short-circuit 
may  feed  into  it  from  both  directions,  and  breakers  will  be  opened 
on  both  lines.  In  this  case,  reverse-load  relays  that  are  instan- 
taneous in  their  action  may  be  installed  in  the  substation.  These 
relays  open  as  soon  as  the  power  begins  to  feed  back,  and  it  is 
then  impossible  for  that  fault  to  open  the  other  feeder.  This 
serves  as  an  outline  of  an  application  of  reverse-load  relays;  some 
difficulties  have  to  be  met,  which  need  not  be  considered  in  this 
discussion. 


CIRCUIT-BREAKING  EQUIPMENT 


141 


A  different  application  of  relays  is  utilized  in  the  Mertz- 
Price  system.  Two  current  transformers,  a  and  6,  Fig.  73,  are 
at  opposite  ends  of  a  transmission  line,  and  their  secondaries 
are  connected  together  by  two  small  wires  running  the  entire 
length  of  the  line,  with  a  relay,  R,  in  their  circuit.  The  connec- 
tions are  such  that  in  normal  operation  the  two  transformers 


Circuit-breakers 

Operated  by 
Overload  Relays 


Circuit-breakers 
Operated  by 
Reverse  Load 
Relays 

Buses  at 
End  of  Line 


Buses  at  End 
of  Line 


FIG.  72. — Parallel  feeders  protected 
by  overload  and  reverse-load  relays. 


FIG.  73. — Mertz-Price  system 
for  protecting  against  short-circuits 
and  grounds. 


oppose  each  other.  If  the  current  at  a  is  the  same  as  that  at  6, 
the  effects  of  the  two  transformers  are  equal,  and  neutralize  each 
other.  But  if  there  is  a  ground  or  short-circuit  at  S,  disturb- 
ing the  balance,  or  reversing  the  current  at  one  end,  a  current 
flows  through  the  relay,  which  closes  a  contact  and  trips  the 
circuit-breaker. 


CHAPTER  XVIII 


Buses 


LIGHTNING-ARRESTER  EQUIPMENT1      . 

When  lightning  strikes  a  line,  it  passes  on  to  ground  by  the 
easiest  path.  If  there  is  no  easier  one,  it  punctures  the  insulation 
of  a  machine  or  transformer,  or  passes  through  the  thin  flanges 
of  a  line  insulator.  The  excessive  voltage  comes  with  such 
extreme  suddenness  as  to  make  it  impracticable  to  operate  a 
mechanism,  connecting  the  line  by  an  easy  path  to  ground.  The 
lightning  discharge  is  equivalent  to  a  high-frequency  current, 

and  on  this  account  a  choke  coil 
located  where  the  line  enters  a  power 
station,  and  connected  in  series  with 
the  line,  prevents  most  of  the  light- 
ning discharge  from  entering  the 
building.  But  this  alone  is  not  suffi- 
cient ;  an  easy  path  to  ground  must  be 
established.  This  path  cannot  exist 
under  normal  operating  conditions, 
because  the  line  would  then  be 
grounded;  but  it  must  be  established 
instantly  when  the  lightning  strikes. 
Two  satisfactory  media  have  been 
found  for  this  purpose:  the  air  gap 
and  the  aluminum  cell.  Either  of 
these,  when  connected  between  line 
and  ground  and  properly  adjusted, 
breaks  down  the  instant  there  is  an 

excessive  voltage  stress.     We  shall  consider  several  arrangements 
of  air  gaps,  and  some  further  facts  about  aluminum  cells. 

A  multigap  arrester,  Fig.  74,  consists  of  a  series  of  metal 
cylinders,  placed  close  together  but  not  touching.  These  little 
cylinders  are  arranged  so  that  each  line  of  the  three-phase  or 

1  G.  372,  374. 

S.  10:850-868;  11:69-80,  220;  12:146-154;  24:733-735. 
A.  pp.  869-872;  360-361. 

142 


^=r  Ground 
Outgoing  or 
Incoming  Feeder 

FIG.  74.— Multigap  lightning 
arrester. 

Disconnecting  switches,  if  used  to 
disconnect  the  lightning  arrester, 
are  inserted  at  D,  D,  D.  For  sim- 
plicity, circuit-breakers  and  discon- 
necting switches  are  omitted  from 
succeeding  diagrams  of  lightning 
arresters.  They  are  connected  as 
shown  here. 


LIGHTNING-ARRESTER  EQUIPMENT 


143 


other  circuit  is  connected  to  ground  through  a  series  of  small  air 
gaps.  The  advantage  of  several  small  gaps  in  series,  instead  of 
one  large  gap,  is  that  after  the  lightning  has  passed,  the  spark 
across  the  little  gaps  does  not  develop  into  an  arc,  which  might 
continue  grounding  and  short-circuiting  the  line;  whereas  with  a 
single  long  gap  the  vapor  of  the  metal  might  produce  a  serious 
persistent  short-circuit.  The  cylinders  of  the  arrester  are  made  of 
a  " non-arcing"  alloy — that  is,  one  that  does  not  tend  to  continue 
the  arc.  This  also  tends  to  quench  the  spark  as  soon  as  the  light- 
ning has  passed.  Every  arrester  must  have  some  such  means  of 
stopping  the  current  from  flowing  to  ground  after  the  lightning 


Outgoing  or 
Incoming  Feeder 

FIG.  75. — Multigap  arrester 
with  resistance  elements  in 
parallel.  Connections  to  the 
other  phases  are  the  same  as 
the  one  shown. 


orn-gaps 


D    Other 
Arrester 


FIG.  76. — Horn-gaps  used  in  con- 
junction with  some  other  lightning 
arrester. 


discharge.  This  type  of  arrester  is  suitable  for  A.C.,  but  not 
D.C.,  and  is  made  for  use  in  conjunction  with  various  arrange- 
ments of  resistances — for  example,  such  as  that  in  Fig.  75 — for 
voltages  ranging  as  high  as  50,000. 

A  horn-gap  arrester,  Fig.  76,  consists  of  a  gap  such  as  would 
be  formed  between  the  two  sides  of  a  V  if  the  bottom  of  the  V  were 
removed.  A  three-phase  arrester  consists  essentially  of  three 
such  horn-gaps,  each  connecting  from  one  line  to  ground;  a 
resistance  rod,  an  aluminum  arrester,  or  some  other  additional 
element  is  put  in  series  with  each  horn-gap,  preventing  a  bad 
short-circuit  which  would  otherwise  occur.  When  the  lightning 
discharge  has  passed,  the  heat  of  the  arc  and  the  magnetic  effect 
of  the  current  tend  to  carry  it  upward  until  it  is  stretched  out 
so  long  that  it  breaks, 


144 


ELECTRICAL  EQUIPMENT 


A  magnetic  blowout  arrester,  Fig.  77,  is  similar  to  a  horn- 
gap,  in  that  there  is  a  gap  in  series  with  another  element, 
and  there  is  a  means  of  blowing  out  the  arc.  In  this  case  the 
series  element  is  a  resistance  rod  made  of  carborundum,  and 
the  blowout  is  an  electromagnet  connected  in  parallel  with  a 
part  of  the  resistance.  This  arrester  is  made  for  all  D.C. 
circuits  up  to  1,500  volts. 


FIG.  77. — Magnetic  blowout 
lightning  arrester. 


FIG.  78. — Condenser 
lightning  arrester. 


A  condenser  arrester,  Fig.  78,  consists  of  a  condenser  in 
parallel  with  a  resistance  rod,  and  these  two  in  series  with  a 
very  small  spark  gap.  The  high-frequency  A.C.  charges  and 
discharges  the  condenser,  with  a  very  low  voltage  to  ground. 
If  there  is  also  a  continuous  charge  (not  alternating),  it  flows 
to  ground  through  the  resistance.  This  is  intended  for  line  and 
car  use  on  D.C.  circuits  up  to  1,500  volts. 


Horn-gaps 
Aluminum  Cell 


FIG.  79. — Aluminum  cell  lightning  arrester,  with  horn-gaps  in  series. 

A  multipath  arrester  is  one  in  which  a  special  composition 
acts  as  a  partial  conductor,  filled  with  minute  spark  gaps.  The 
relatively  large  mass  of  the  substance  absorbs  the  heat  of  the 
minute  sparks;  and  as  soon  as  the  lightning  discharge  stops,  the 


LIGHTNING-ARRESTER  EQUIPMENT  145 

path  to  ground  ceases  to  conduct  the  current.  This  arrester  is 
intended  for  use  on  400-  to  750-volt  D.C.  and  A.C.  circuits. 

An  aluminum  arrester,  Fig.  79,  is  made  up  of  a  series  of  alumi- 
num pans,  each  inside  of  the  one  below,  and  each  filled  with  an 
electrolytic  solution,  and  immersed  in  oil.  The  surfaces  of  the 
aluminum  pans  are  covered  with  a  film  of  aluminum  hydroxide, 
which  acts  almost  as  an  insulator  until  the  voltage  reaches  a  cer- 
tain value.  But  when  the  voltage  becomes  excessive,  the  film 
breaks  down,  and  the  whole  arrester  becomes  a  good  conductor. 
And  as  soon  as  the  voltage  drops  again,  the  insulating  film  is 
again  restored.  This  type  of  arrester  is  commonly  used  in  con- 
junction with  a  horn-gap,  which  has  the  effect  of  insulating  the 
arrester  from  the  line,  except  during  the  lightning  discharge. 
This  arrester  is  adapted  for  all  voltages  from  2,000  up,  and  for 
lower  voltages  on  D.C. 

Relative  Merits  of  Arresters. — In  selecting  a  lightning  arrester, 
the  line  voltage  of  the  system  is  to  be  considered  first,  then  the 
severity  of  the  lightning  and  the  total  current  capacity  of  the 
generators  in  the  vicinity;  because  large  generators  may  be  capa- 
ble of  pouring  a  large  current  through  the  arrester  to  ground, 
after  the  lightning  strikes,  before  normal  conditions  are  again 
established.  The  ideal  arrester  acts  on  an  electric  circuit  as  a 
good  safety  valve  does  on  a  steam  boiler.  When  there  is  a  slight 
excess  in  voltage,  the  current  flows  freely  to  ground,  through  the 
equivalent  of  a  low  resistance;  but  as  soon  as  the  voltage  becomes 
normal,  the  current  to  ground  is  cut  off  by  an  increase  in  the 
equivalent  resistance. 

The  aluminum  arrester  performs  its  function  much  better  than 
any  other  does  on  very  high  voltages;  and  it  is  recommended  as 
the  preferable  arrester  to  apply  on  voltages  even  as  low  as  2,000. 
For  D.C.  circuits  of  even  lower  voltage  it  is  of  advantage,  where 
the  lightning  is  very  severe.  The  reason  for  the  success  of  the 
aluminum  arrester  is  that  the  resistance  is  extremely  low  to  the 
high  voltage  of  the  lightning,  but  it  is  extremely  high  as  soon  as 
the  lightning  has  passed.  The  disadvantage  of  the  aluminum 
arrester  is  that  it  requires  a  little  attention.  It  should  be 
"  charged"  every  day — that  is,  connected  across  the  line  without 
an  air  gap  in  series — to  keep  the  insulating  film  in  good  condi- 
tion. The  cost  is  higher  for  aluminum  arresters  than  for  suitable 
types  of  spark-gap  arresters,  especially  for  low-voltage  circuits. 

Choke  coils  are  put  in  series  with  the  line,  as  it  enters  or  leaves 
10 


146 


ELECTRICAL  EQUIPMENT 


a  building,  to  keep  the  lightning  out.  They  are  coils,  wound 
without  iron,  and  having  a  quite  low  inductance.  Being  in 
series  with  the  line,  they  introduce  a  reactance,  2irfL,  where 
/  is  the  line  frequency  and  L  the  inductance  of  the  coil.  The 
inductance  is  so  low  that  the  drop  is  almost  negligible  in  normal 
operation;  but  the  lightning  is  equivalent  to  an  extremely  high- 
frequency  current,  and  the  opposition  offered  to  the  lightning  is 
correspondingly  great. 

Choke  coils  differ,  first,  as  to  current-carrying  capacity,  second, 
as  to  the  voltage  for  which  they  are  insulated  from  ground,  and 
third,  as  to  their  inductance.  An  increase  in  any  of  these  increases 
the  cost  of  the  choke  coil.  The  current  and  voltage  capacities 
must  be  those  of  the  line.  The  larger  the  inductance,  the  more 
effective  is  the  choking  in  keeping  the  lightning  out  of  the  build- 
ing. On  short  lines  of  low  voltage,  the  importance  of  choke  coils 
is  relatively  small — in  fact,  sometimes  both  choke  coils  and  ar- 
resters may  be  omitted — but  on  voltages  from  2,200  up  on  long 
lines,  subject  to  severe  lightning  disturbances,  choke  coils  and 
arresters  are  important.  A  choke  coil  may  be  considered  as  equi- 
valent to  a  multiplier,  increasing  the  effectiveness  of  the  arrester. 
It  can  be  omitted  where  the  arrester  is  adequate  without  it. 

-Ground  Wire 


Outgoing  or 
Incoming  Feeder 


FIG.  80. — Relative  connections 
of  lighting  arrester,  choke  coils  and 
circuit-breaker. 

Disconnecting  switches  at  D,  D,  D,  if  used. 


FIG.  81. — Ground  wire,  above 
the  line  wires  L,  L,  L. 


The  larger  the  inductance  of  a  choke  coil,  the  more  must  one 
end  of  the  coil  be  insulated  from  the  other;  for  when  the  lightning 
strikes,  the  pressure  exerted  may  be  about  proportional  to  the 
inductance.  Sometimes  choke  coils  of  heavy  inductance,  in- 
tended for  use  on  high-tension  lines,  are  immersed  in  oil  for 
better  insulation. 


LIGHTNING-ARRESTER  EQUIPMENT  147 

Fig.  80  shows  a  suitable  arrangement  of  choke  coils  and  ar- 
resters, connected  to  a  line  entering  a  power  station. 

Ground  Connections. — The  path  to  ground  must  be  good, 
to  insure  proper  operation  of  the  arrester.  It  should  have  as 
few  turns,  and  as  little  horizontal  length,  as  possible.  It  should 
go  down  to  moist  earth,  and  should  have  exposed  there  several 
square  feet  of  surface,  consisting  of  sheet  copper,  iron  pipes  or 
rods,  or  other  adequate  grounding  surface. 

If  the  ground  wire  is  continued  along  the  transmission  line, 
over  the  other  wires,  as  in  Fig.  81,  it  helps  to  shield  the  other  wires 
and  the  arrester  from  lightning  disturbances.  This  would  be 
an  unnecessary  expense  in  some  cases,  but  in  others  it  is  of 
material  value. 


CHAPTER  XIX 
MEASURING  AND  INDICATING  APPARATUS 

This  chapter  is  treated  under  four  heads: 

1.  Meters  and  the  quantities  measured. 

2.  Characteristics  of  meters. 

3.  Meter  switching  devices. 

4.  Meter  applications. 

METERS  AND  THE  QUANTITIES  MEASURED 

The  instruments  that  we  consider  are  those  in  common  use 
in  commercial  and  industrial  plants,  for  indicating  current, 
voltage,  frequency,  grounds,  and  single-phase  and  polyphase 
power,  energy  and  power  factor.  Some  of  them  are  so  well 
known  as  to  require  little  or  no  description. 


FIG.  82. — Diagram  of  polyphase  wattmeter. 

Current  circuit  Ii  and  voltage  circuit  Ei  comprise  one  meter  element;  circuits  1 2  and  Ez 
comprise  the  other  element.  In  induction  types  of  meters  both  the  current  and  the  voltage 
circuits  are  stationary.  The  currents  in  these  stationary  circuits  induce  eddy  currents 
in  rotable  disks  or  drums,  and  these  eddy  currents  react  with  the  magnetic  field,  rotating 
the  moving  element. 

A  polyphase  wattmeter  consists  of  two  single-phase  meter 
elements  that  are  electrically  complete  and  distinct;  they  act 
on  a  single  pointer,  tending  to  deflect  it,  and  the  deflection  is 
opposed  by  a  spring.  The  connections  are  illustrated  in  Fig. 
82.  The  scale  indication  is  equivalent  to  that  of  two  single- 
phase  wattmeters.  It  is  at  once  evident  that  two-phase  power 
can  be  measured  with  this  meter,  by  connecting  one  phase  to 

148 


MEASURING  AND  INDICATING  APPARATUS      149 

one  wattmeter  element  and  the  other  phase  to  the  other  element. 
It  has  been  shown  in  a  variety  of  ways  that  three-phase  power 
can  be  similarly  measured.1 

A  watt-hour  meter  (formerly  called  an  integrating  wattmeter 
or  a  recording  wattmeter)  makes  a  continuous  record,  summing 
up  the  total  energy  that  has  flowed  since  any  given  time,  so 
that  by  reading  dial  indications  every  day,  month,  or  other  period, 
the  energy  used  in  each  period  is  obtained  directly.  Watt-hour 
meters  are  either  single-phase  or  polyphase.  The  polyphase 


+x 


FIG.  83. — Diagram  showing  the  principle  of  operation  of  a  single-phase 
power-factor  meter. 

Winding  Z  is  a  current  winding;  it  is  connected  either  in  series  with  the  line  or  to  the 
secondary  of  a  current  transformer  whose  primary  is  in  series  with  the  line. 

Windings  +  R  and  —  R  are  voltage  windings  with  resistance  in  series. 

Windings  -f-  X  and  —  X  are  voltage  windings  with  reactance  in  series.  These  windings 
are  connected  either  across  the  main  circuit  or  to  the  secondary  of  a  voltage  transformer 
whose  primary  is  across  the  circuit. 

In  polyphase  meters,  sometimes  winding  I  is  a  voltage  winding,  connected  across  the 
circuit,  and  windings  +  R,  —  R,  +  X,  —  X,  or  similar  windings  are  current  windings, 
connected  in  series  with  the  several  phases.  The  resistance  and  reactance  are  then  omitted. 

In  one  important  type  of  power  factor  meter,  winding  J  is  stationary,  but  the  form  of  the 
coil  and  the  iron  core  are  such  that  the  core  can  rotate  as  required  to  indicate  the  power 
factor. 

watt-hour  meter  consists  of  two  single-phase  elements,  arranged 
as  in  wattmeters. 

Power-factor  meters,  or  power-factor  indicators,  usually  have 
two  fields,  one  produced  by  the  line  current  and  the  other  by  the 
line  voltage.  Either  the  current  or  the  voltage  field  is  made 
rotating,  the  other  being  simply  an  alternating  field.  The  mov- 
ing element  of  the  meter  is  made  so  that  it  is  deflected  to  a  posi- 

1  Compare  G.  267,  Fig.  271,  which  applies  to  one  polyphase  meter,  as 
well  as  to  two  single-phase. 
S.  3:  171-177. 
A.  p.  1825. 


150  ELECTRICAL  EQUIPMENT 

tion  that  is  determined  by  the  relation  of  the  current  and  voltage 
fields.  These  meters  can  be  obtained  for  single-phase,  two- 
phase  and  three-phase  circuits. 

The  principle  of  operation  (but  not  the  form  of  the  instru- 
ment) of  a  single-phase  power-factor  meter  is  illustrated  in  Fig. 
83.  The  four  stationary  poles,  +R,  -\-X,  —  R,  —X,  are  excited 
by  windings  connected  across  the  voltage,  with  reactance  and 
resistance  respectively  in  circuit,  as  indicated.  The  combined 
effect  of  these  four  windings  is  to  produce  a  rotating  field.  The 
central  rotating  electromagnet  is  excited  by  a  winding,  7,  which 
is  connected  in  series  with  the  line,  and  carries  the  line  current. 
It  is  attracted  to  the  position  in  which  the  rotating  field  of  -\-R, 
-\-X,  —R,  —X  is  in  phase  with  the  current  in  7.  The  angular 
position  of  the  moving  element  is  shown  by  the  pointer,  and 
with  suitable  calibration  it  indicates  the  power  factor. 

With  some  variation,  this  discussion  applies  to  all  types  of 
power-factor  meters,  for  single-phase  and  polyphase  circuits. 

Synchronism  indicators,  or  synchronoscopes,  are  made  on  the 
same  principle  as  power-factor  meters,  except  that  windings 
-\-R,  -\-X,  —R,  —X  are  voltage  windings  connected  across  the 
"running"  machine,  and  winding  7  is  a  voltage  winding  connected 
across  the  "starting"  machine.1  The  pointer  takes  a  position 
depending  on  the  relation  of  these  two  fields.  If  one  is  gaining 
on  the  other,  or  losing,  the  fact  is  indicated  by  a  rotation  of  the 
pointer,  to  the  right  or  left.  This  rotation  is  very  slow  when 
the  starting  machine  has  about  the  right  speed.  When  the 
pointer  is  stationary,  or  nearly  so,  pointing  vertically  upward, 
the  machines  are  exactly  or  approximately  in  synchronism,  and 
the  switch  of  the  incoming  machine  may  be  closed. 

Lamps  for  Synchronizing. — Lamps  may  be  connected  in  series 
between  the  two  machines,  so  as  to  show  by  their  brilliancy 
when  the  machines  are  in  phase.  With  the  customary  arrange- 
ment of  connections,  when  the  lamps  go  out  the  machines  are 
in  synchronism.  The  most  approved  method  of  synchronizing 
is  by  using  both  the  lamps  and  the  synchroscope.2 

xThe  terms  "running"  machine  and  "starting"  machine  refer  to  the 
machine  already  connected  to  the  buses  and  the  one  to  be  synchronized. 
Sometimes  the  starting  machine  is  referred  to  as  "incoming." 

2  HAKOLD  W.  BROWN,  "Apparatus  for  Synchronizing,"  The  Electric 
Journal,  vol.  v,  p.  530,  September,  1908. 

HAROLD  W.  BROWN  and  S.  S.  NEU,  "Phasing  out  for  Synchronizing 
Polyphase  Circuits,"  The  Electric  Journal,  vol.  ix,  p.  427,  May,  1912. 


MEASURING  AND  INDICATING  APPARATUS      151 

Frequency  meters  are  of  two  distinct  kinds.  One  is  similar 
to  a  differential  voltmeter,  in  that  it  has  two  voltmeter  elements 
opposed  to  each  other,  tending  to  deflect  the  pointer  in  opposite 
directions.  As  in  Fig.  84,  both  of  the  voltage  windings  are 
connected  across  the  same  circuit,  so  that  if  each  had  the  same 
impedance  in  series  with  it  the  meter  would  always  indicate 
zero.  One  winding  has  a  large  resistance  and  no  reactance  in 
series  with  it,  so  that  its  current  at  a  given  voltage  is  the  same 
at  all  frequencies;  the  other  winding  has  a  large  reactance  and 
small  resistance,  and  of  course  the  reactance  varies  directly  with 
the  frequency.  Thus  the  element  having  the  large  reactance 
is  weak  at  high  frequencies,  but  strong  at  low  frequencies;  so 


FIG.  84. — Connections  of  re- 
sistance- and-reactance  fre- 
quency meter. 

The  resistance  element  exerts  a 
torque  that  is  independent  of  fre- 
quency; the  reactance  element  exerts  a 
torque  that  decreases  as  the  frequency 
is  increased. 


FIG.  85. — Electrostatic  ground 
detector. 


that  with  the  right  calibration  the  pointer  indicates  the  different 
frequencies. 

The  other  kind  of  frequency  meter  has  a  series  of  vibrating 
reeds  each  tuned  for  a  different  frequency.  A  coil  is  placed  in 
such  a  position  that  it  tends  to  make  all  the  reeds  vibrate,  and 
the  frequency  is  indicated  by  the  one  having  the  greatest  vibra- 
tion. 

A  ground  detector  is  sometimes  made  in  the  form  of  a  differen- 
tial electrostatic  voltmeter — that  is,  an  electrostatic  voltmeter 
which  shows  by  its  deflection  if  the  voltage  from  one  line  to 
ground  is  greater  than  from  another  to  ground.  One  form  is 
illustrated  in  Fig.  85.  - 


152  ELECTRICAL  EQUIPMENT 

A  ground  detecting  lamp  may  be  connected  from  each  line  to 
ground.  If  one  lamp  goes  out  or  burns  dim,  it  indicates  that 
the  corresponding  line  is  grounded.  This  is  illustrated  in  Fig. 
86  (see  also  Fig.  99,  p.  161). 


D.dor  Single  Phase  3  Phase  2  Phase 


FIG.  86. — Arrangement  of  ground  detecting  lamps  on  D.C.,  single-phase  and 

polyphase  circuits. 


CHARACTERISTICS  OF  METERS 

There  is  a  great  variation  in  the  characteristics  of  the  various 
kinds  of  meters;  the  accuracy  depends  on  the  calibration  and 
construction  of  the  meter,  and  in  part  on  whether  the  meter  is 
used  under  exactly  the  conditions  for  which  it  was  made.  Some 
meters  can  be  used  indiscriminately  under  widely  varying  con- 
ditions; others  are  subject  to  considerable  errors1  due  to  excess- 
ively high  or  low  temperature,  mechanical  balance,  friction, 
aging,  distorted  wave  form  of  current  or  voltage,  thermo-elec- 
tromotive  forces,  and  perhaps  a  few  other  causes,  which  are  more 
or  less  beyond  the  control  of  the  user,  but  should  be  specified 
in  purchasing  meters.  In  addition  to  these  are  several  features 
and  conditions  causing  errors,  as  mentioned  below,  which  should 
be  considered  in  both  purchasing  and  using  meters: 

The  scale  of  a  meter  may  have  equally  spaced  divisions,  such 
as  are  in  most  of  the  permanent  magnet  types  of  voltmeters 
and  ammeters,  or  the  spaces  may  be  wide  near  the  middle  of  the 
scale  and  narrow  at  each  end,  or  they  may  increase  gradually, 
so  that  they  are  widest  at  the  end  of  the  scale.  These  are 
illustrated  in  Fig.  87.  Where  it  is  possible,  the  best  meter  for 
all-round  use  has  a  uniform  scale,  but  if  readings  are  nearly 
always  taken  in  a  certain  part  of  the  scale,  there  is  a  possible 
advantage  in  having  the  divisions  wider  in  that  part.  Also, 
in  general,  the  longer  the  scale  the  smaller  will  be  the  reading 
error. 

1  CYRIL  JANSKY,  "Electric  Meters,"  p.  345  (New  York:  McGraw-Hill 
Book  Co.,  Inc.),  First  Edition,  1913. 


MEASURING  AND  INDICATING  APPARATUS      153 

A  meter  should  be  selected,  preferably,  of  such  a  full-scale  in- 
dication that  in  ordinary  use  the  indications  are  beyond  the  mid- 


70       so 


FIG.  87. — Typical  voltmeter  or  ammeter  scales. 

(a)  Uniform  scale  divisions.     (6)  Wide  divisions  near  middle  of  scale,     (c)   Wide  divi- 
sions near  end  of  scale. 

die  of  the  scale.  Accuracy  is  sacrificed  in  taking  readings  at 
much  less  than  one-half  of  full  scale.  For  example,  a  current 
of  50  amp.  cannot  be  measured  on  a  200-amp.  ammeter  with 


154 


ELECTRICAL  EQUIPMENT 


the  same  per  cent,  accuracy  with  which  150  amp.  can  be 
measured. 

Any  meter  may  have  more  than  one  set  of  terminals  or  con- 
nections, by  which  the  meter  can  be  made  to  indicate  either 
large  or  small  quantities,  as  illustrated  in  Fig.  87a.  It  is  then 
convenient  to  have  the  numbers  on  the  two  scales  in  different 
colors,  to  agree  with  markings  on  the  corresponding  meter  ter- 
minals. If  two  sets  of  numbers  are  shown  on  the  scale,  such  a 
meter  is  called  a  "  double-scale "  meter. 

In  some  cases  meters  are  required  to  read  both  positive  and 
negative  quantities — e.g.,  incoming  and  outgoing  kilowatts. 
They  are  then  provided  with  a  scale  extending  both  to  t'~e  right 


FIG.  88. — Scales  with  Shifted  zero. 

(a)   Zero-center  scale,     (b)  Suppressed-zero  scale. 

and  to  the  left  of  zero,  as  in  Fig.  88a.  Such  a  meter  is  called  a 
"  double-reading "  meter,  or  the  scale  a  "  zero-center "  scale. 
In  certain  meters  a  zero  reading  is  never  required,  and  relatively 
wide  scale  divisions  are  desirable.  The  zero  may  then  be  "  sup- 
pressed, "  as  in  Fig.  886.  This  has  the  disadvantage,  however, 
that  it  is  less  convenient  to  adjust  the  pointer  if  an  error  on  zero 
is  introduced  by  a  bent  pointer  or  in  any  other  way. 

Frequency  affects  some  kinds  of  meters,  and  not  others.  In 
general,  it  has  little  effect  on  meters  of  the  dynamometer  and 
Kelvin  balance  type.  The  effect  of  frequency  on  some  induction- 
type  meters  is  greater  than  on  others.  The  fact  that  meters 
are  nearly  always  used  on  either  25-  or  60-cycle  circuits,  and  that 
the  variation  from  the  normal  frequency  is  very  slight,  makes 


MEASURING  AND  INDICATING  APPARATUS      155 

the  disturbance  due  to  frequency  rather  insignificant  in  most 
cases. 

Voltage. — A  change  of  the  magnetic  condition  of  the  iron  with 
voltage  may  effect  the  accuracy  of  a  wattmeter,  or  a  watt-hour 
meter.  It  may  also  effect  an  ammeter  whose  field  is  produced 
by  an  electromagnet.  Voltage  variation  may  effect  the  resist- 
ance-and-reactance  type  of  frequency  meter,  because  both  of 
the  opposing  elements  are  weaker  at  low  voltage.  Usually  all 
these  effects  are  negligible  when  the  meter  is  used  within  10  per 
cent,  of  the  rated  voltage. 

Low  power  factor  has  no  effect  on  the  accuracy  of  any  meters 
except  wattmeters  and  watt-hour  meters.  If  even  these  meters 
are  properly  adjusted  for  power  factor  it  should  have  no  effect 
on  them. 

Unbalancing  of  phases  of  a  polyphase  wattmeter  or  watt-hour 
meter  should  have  no  effect  if  the  two  elements  of  the  meter  are 
independently  correct  at  high  and  low  power  factors  and  there 
is  no  stray-field  effect  of  one  element  of  the  meter  on  the  other. 

A  stray-field  may  affect  the  accuracy  of  a  meter  if  it  has  the 
same  frequency  as  the  quantity  that  the  meter  indicates,  or 
both  the  field  and  the  quantity  measured  are  from  a  D.C.  source. 
A  D.C.  ammeter  or  voltmeter  should  not  be  too  near  a  D.C. 
conductor  carrying  a  heavy  current.  Strong  A.C.  fields  are  not 
so  likely  to  be  near  the  A.C.  meters,  but  they  also  should  be 
avoided  unless  they  are  known  to  have  a  negligible  effect.  Some 
switchboard  and  other  meters  have  iron  cases,  which  shield 
them  very  largely  against  such  magnetic  disturbances. 

Instrument  transformers,  including  both  current  and  voltage 
transformers,  have  negligible  errors  if  they  are  not  furnishing 
power  to  too  many  instruments;  but  if  the  number  of  instruments 
is  too  great,  considerable  errors  are  introduced,  both  in  ratio 
and  in  " phase  displacement"  (i.e.,  phase  error).  Ammeters, 
voltmeters  and  other  apparatus  operating  on  current  or  voltage 
'are  affected  by  ratio  errors;  wattmeters  and  watt-hour  meters 
are  affected  by  both  ratio  and  phase  displacement.1  The  effect 
of  current  transformers  on  the  accuracy  of  watt-hour  meter 
indications  is  illustrated  in  Figs.  89  to  91,  in  which  typical  instru- 
ments are  connected  to  current  transformers  that  are  compen- 
sated for  a  secondary  load  (i.e.,  load  due  to  the  meter  winding) 

J  See  Chapter  XV,  "Current  Transformers." 


156 


ELECTRICAL  EQUIPMENT 


of  25  volt-amp.1    In  each  of  these  figures,  six  curves  are  drawn. 
Three  of  them  show  the  error  in  the  watt-hour  meter  itself,  when 


fl. 

li 


Accuracy  Curves 

Current  Transformer  win  S ingle - 
Pnose  Watt  hour  Meter  Only. 
1007.  Loaa*5 Amperes. 
60  Cycles 


Primary 


I — ^WW 1 


130    140 


FIG.  89. — Typical  accuracy  curves,  showing  the  errors  at  various  loads 
and  power  factors,  when  a  single  phase  watt-hour  meter  is  connected  directly 
and  through  a  current  transformer  to  a  single-phase  60-cycle  circuit. 


Accuracy  Curves 

Current  Transformer  with  Poly- 
Phase  Wat  I  hour  Meter  Only 
100%  Load -5  Amperes. 
60  Cycles 


FIG.  90. — Same  as  Fig.  89,  except  that  a  polyphase  watt-hour  meter  is  con- 
nected to  a  £/wee-phase  circuit. 

the  load  is  respectively  at  50,  75,  and  100  per  cent,  power  factor. 

1  The  meaning  of  "a  secondary  load  of  25  volt-amp."  is  somewhat 
arbitrary,  referring  to  the  secondary  volt-ampere  output  of  the  current 
transformer  at  rated  full-load  current. 

A  watt-hour  meter  is  a  load  of  about  2  volt-amp. 

An  ammeter  is  a  load  of  about  5  volt-amp. 

A  trip-coil  is  a  load  of  about  50  volt-amp. 


MEASURING  AND  INDICATING  APPARATUS      157 


The  other  three  curves  in  each  figure  show  the  combined  error 
of  the  wattmeter  and  the  current  transformer.  The  difference 
between  the  two  curves  shows  the  transformer  error.1  Fig.  89 
shows  that  on  a  single-phase  system,  if  nothing  but  the  watt- 
hour  meter  is  connected  to  the  transformer,  the  transformer 
error  is  practically  zero  at  light  load,  rising  to  about  0.5  per  cent. 


Accuracy  Curves 

Current  Transformer  with  Poly  - 
Phase  Watt  hour  Meter,  Ammeters. 
and  Trip  Coils. 

100%  Load  '5  Amperes 
60  Cycl 


Cycles. 


J 

ppy 


(0     10     30     40     50 


110    IZO    130    140 


FIG.  91. — Same  as  Fig.  90,  except  that  ammeters  and  circuit-breaker 
trip-coils  are  connected  in  series  with  the  current  transformers. 

When  the  trip-coils  are  inserted,  note  the  relatively  large  errors  with  light  load  at  all  power 
factors,  and  with  full  load  at  50  per  cent,  power  factor.  This  is  in  spite  of  the  very  high 
quality  of  design  and  construction  of  this  type  of  transformer. 

at  30  per  cent,  overload,  depending  a  trifle  on  the  power  factor. 
Fig.  90  shows  that  on  a  three-phase  system,  with  a  polyphase 
watt-hour  meter,  the  errors  are  of  about  the  same  order,  except 
that  there  is  a  negative  error  of  about  1  per  cent,  at  light  loads. 
Fig.  91  shows  that  if  trip-coils  and  ammeters  are  connected  in 
series  with  the  watt-hour  meter,  this  negative  error  at  light 
load  is  considerably  increased,  but  there  is  a  larger  positive  error, 

*The  curves  were  furnished  by  courtesy  of  the  Westinghouse  Electric 
and  Manufacturing  Co.  and  represent  tests  on  Westinghouse  equipment. 


158 


ELECTRICAL  EQUIPMENT 


at  large  loads  and  low  power  factor.  (This  error  is  chiefly  due 
to  the  trip-coils — not  to  the  ammeters.)  The  conclusion  to  be 
reached  from  these  curves  is  that  where  considerable  accuracy 
is  required1  at  light  load  or  low  power  factor,  the  watt-hour 
meter  should  not  be  put  on  current  transformers  that  operate 
trip-coils. 


Typical  Ratio 

Phase  Displacement  Curves. 
Type  "A' 'Current  Transformer. 

Secondary  Load  at  5  Amps. 
2*4  Volt-Amps,  at  JJ7.  P.  fl- 60  Cycles. 
2-10.     -  •     "      •  90%   "  -  • 


FIG.  92.  FIG.  93. 

(Curves  are  for  60-cycles.)  (Curves  are  for  25  and    60  cycles.) 

The  advantage   of  this  transformer  is  in  The  advantage  of  this  transformer  is 

its  small  errors.  that  its  cost  is  only  two-thirds  the  cost  of 

the  transformer  of  Fig.  92. 

FIG.  92. — Transformer  of  50  volt-amperes  capacity,  compensated  for 
25  volt-amperes. 

Fig.  93. — Transformer  of  10  volt-amperes  capacity,  compensated  for 
10  volt-amperes. 

FIGS.  92  and  93. — Ratio  and  phase  displacement  curves  of  current  trans- 
formers. 

The  meaning  of  101  per  cent,  ratio  is  that  primary  current/secondary  current  is  101  per 
cent,  of  the  correct  (rated)  ratio. 

The  meaning  of  60  minutes  phase  displacement  is  that  the  secondary  current  is  1  degree 
(1/360  of  a  cycle)  ahead  of  the  primary.  If  the  primary  has  a  lagging  current,  this  dis- 
placement tends  to  make  a  wattmeter  or  watt-hour  meter  reading  too  high,  whereas  the  ratio 
error  tends  to  make  it  top  low;  so  that  the  two  errors  tend  to  neutralize  each  other. 

The  effect  of  phase  displacement  on  wattmeter  and  watt-hour  meter  indications  is  as 
follows: 

1.00  f  0.02  of  1 

0.90        The  error  per  cent,  introduced  per  \  0.85  of  1 
0.80  degree  of  phase  displacement  is  }  1.3 

0.70  1.7 


With    a    lagging    current 
having  a  power  factor  of 


Fig.  92  shows  ratio  and  phase  displacement  curves  of  a  trans- 
former such  as  was  used  in  Figs.  89  to  91,  under  various  condi- 
tions of  loading,  with  various  instruments  connected  to  the 
transformer  secondary.  Fig.  93  is  similar  to  Fig.  92,  but  refers 
to  a  transformer  of  10  volt-amp,  secondary  capacity. 

1  For  example,  where  charges  for  electric  energy  are  made  from  the 
meter  readings. 


MEASURING  AND  IN  DIG  A  TING  APPARA  TUS      159 


Beceptacles 


METER  SWITCHING  DEVICES 

The  number  of  meters  required  in  a  plant  is  very  much  reduced, 
if  suitable  plugging  or  other  switching  devices  are  employed  for 
shifting  some  of  the  meters  from  circuit  to  circuit,  or  from  phase 
to  phase  of  a  circuit.  Follow- 
ing are  a  few  of  these  devices: 

Four-point  Voltmeter  Plugs 
and  Receptacles. — The  recepta- 
cle is  merely  four  terminals,  to 
each  of  which  a  permanent  and 
a  plug  connection  can  be  made. 
Fig.   94  shows  a  suitable  ar- 
rangement of  connections.     Of  Frora  Generators 
the  three  receptacles,  one  con-  for  measuring  generator  andfbus  volt- 
nects  to  the  buses  and  two  to  age  on  a  two-wire  system, 
generators.    All  the  receptacles 

connect  to  the  one  voltmeter.  The  plug,  when  inserted  in  any 
receptacle,  puts  the  voltage  of  the  buses  or  of  one  of  the  gene- 
rators on  the  voltmeter.  It  is  essential  that  only  one  plug  be 


.     1 

lAAAA/v\A 
/vw\jwv\ 

JM 

rr  r 

LO    c~ 

1—  ok> 

(6) 

FIG.  95. — Eight-point  and  six-point  receptacles. 

(o)  Eight-point  receptacle  for  measuring  voltage  on  three-phase  circuits;  may  also  be 
used  on  D.C.  or  single-phase,  three-wire  circuits,  (b)  Same  as  (o)  where  voltage  trans- 
formers are  used,  (c)  Six-point  receptacle  for  measuring  voltage  on  two-phase  circuit. 
(d~)  Same  as  (c)  where  voltage  transformers  are  used. 

provided  for  the  entire  plant ;  because  if  there  were  two,  inserted 
by  mistake  in  different  receptacles  at  the  same  time,  they  might 
introduce  a  short-circuit  between  machines. 


160 


ELECTRICAL  EQUIPMENT 


^Erom  Generators'* 


macl 


Eight-point  Receptacles. — Fig.  95,  a  and  6,  shows  how  an 
eight-point  receptacle  may  be  used  on  a  three-phase  circuit  just 

as  the  four-point  is  used  in 
Fig.  94,  on  B.C.  or  single- 
phase.  A  four-point  plug  is 
used  as  before;  there  are 
three  possible  positions  for 
the  plug  in  each  receptacle, 
for  measuring  the  voltage  of 
the  three  phases.  In  Fig.  95 

FIG.  96.— Synchronizing  plugs  and    c  and  d,  a  six-point  receptacle 
-ironizing  "between    ig  uged   Qn   ft   two.phase   cir- 
cuit. 

Synchronizing  Plugs  and  Receptacles. — Various  plugs  and 
receptacles,  such  as  those  just  described,  are 
used  for  connecting  the  machines  to  the  syn- 
chronism indicator.  One  arrangement  is  shown 
in  Fig.  96,  in  which  there  are  two  kinds  of  plugs 
— one  for  the  starting  and  one  for  the  running 
machine.  The  difference  between  the  plugs  is 
such  that  the  starting  machine  connects  to  the 
top  of  the  synchronism  indicator,  and  the  run- 
ning machine  to  the  bottom.  The  diagram 
shows  one  of  several  synchronism  indicators 
that  are  on  the  market.  The  order  of  leads  is 
different  in  different  types,  but  the  method  of  switch. 
synchronizing  is  essentially  the  same.  Some 
engineers  prefer  to  connect  the  lower,  or  running  leads  of  the 
synchronism  indicator  to  the  buses  in  all 
cases,  instead  of  connecting  to  one  partic- 
ular machine.  The  arrangement  of  wiring 
is  then  somewhat  different,  but  the  results 
are  essentially  the  same.  This  is  illustrated 
in  Fig.  103.  In  the  one  case  they  synchro- 
nize " between  machines"  and  in  the  other 
to  the  buses. 

Rotating   Ammeter   Switch. — Switching 
devices  for  use  with  current  transformers 
and   ammeters   should  be  constructed  so 
that  the  ammeter  can  be  inserted  in  any  phase,  without  opening 
the  circuit  of  any  current  transformer.     Such  devices  are  made 


FIG.    98. — Ammeter 
plug  and  receptacles. 


MEASURING  AND  INDICATING  APPARATUS      161 

on  two  different  plans,  illustrated  in  Figs.  97  and  98.  A  drum 
switch  is  made  in  several  forms  similar  to  Fig.  97.  With  the 
switch  in  the  position  shown,  the  currents  from  current  trans- 
formers A  and  C,  entering  the  switch  at  terminals  1  and  3,  must 
flow  through  the  ammeter  before  they  can  return.  Since  the 
resultant  of  the  A  and  C  currents  is  the  B  current  (see  Chapter 
XV,  p.  119),  the  ammeter  must  now  be  indicating  that  current; 
but  if  the  switch  is  rotated  so  that  the  small  segment  of  the  drum 
bridges  from  1  to  2,  only  the  C  current  flows  through  the  ammeter; 
and  by  rotating  the  switch  in  the  other  direction  the  A  current 
is  indicated. 

Ammeter  Plugs  and  Receptacles. — The  other  device,  which  is 
represented  in  Fig.  98,  consists  of  one  plug  and  as  many  recep- 
tacles as  there  are  lines  in  which  the  current  is  to  be  measured. 


Push 


Indicating  Lamp 
(or  Voltmeter) 

FIG.  99. — Ground  detecting  switches. 

The  plug  is  inserted  across  1-4,  2-4,  or  3-4,  connecting  one  end  of  the  voltage  transformer 
primary  to  any  one  of  the  three  line  wires.  If  the  ground  detecting  push  is  pressed,  it  makes 
contact  at  g  and  connects  the  other  end  of  the  primary  to  ground.  The  indicating  lamp  in 
the  transformer  secondary  will  be  dark,  or  at  most  only  dimly  lighted,  if  connection  is  made 
to  a  line  wire  that  is  grounded. 

When  the  push  is  not  depressed,  a  spring  presses  the  movable  contact  against  a,  con- 
necting to  line  1 .  If  the  plug  is  in  position  2-4,  or  3-4,  the  lamp  then  indicates  full-line  volt- 
age by  its  brilliancy. 

In  case  a  little  better  indication  is  required  in  measuring  ground  voltages,  the  indicating 
lamp  is  replaced  by  a  voltmeter. 

The  construction  of  plugs  and  receptacles  is  not  exactly  according 
to  the  diagram,  which  is  intended  to  show  the  principle,  rather 
than  the  form  of  the  apparatus.  As  the  plug  is  inserted,  it 
first  connects  its  two  contact  surfaces  to  the  ammeter,  and  then 
to  the  transformer;  and  after  that  the  lower  path  of  the  current 
is  opened,  forcing  the  current  to  flow  up  through  the  ammeter. 

This  device  is  sometimes  preferred  to  the  rotating  switch,  as 
by  use  of  a  special  plug  it  permits  connecting  portable  meters 
in  the  circuit  to  check  the  switchboard  instruments. 

Ground  detector  switches  are  made  in  a  variety  of  forms. 
One  arrangement  of  switches  and  a  transformer  is  illustrated  in 
Fig.  99,  by  which  the  voltage  from  any  phase  to  ground  is  indi- 
cated roughly  by  a  lamp  or  a  little  better  by  a  voltmeter.  Only 
11 


162 


ELECTRICAL  EQUIPMENT 


a  110-volt  lamp  or  voltmeter  is  required,  if  it  is  connected  to  the 
line  through  a  voltage  transformer,  as  shown. 


METER  APPLICATIONS 

Meter  Equipment  for  D.C.  Switchboards. — Only  one  volt- 
meter is  necessary  for  the  entire  board.  One  four-point  plug 
should  be  furnished  and  there  should  be  a  receptacle  for  each 
generator.  One  receptacle  in  addition  may  be  provided  to 
measure  bus  voltage,  if  desired.  This  is  illustrated  in  Fig.  94. 

One  ammeter  is  necessary  for  each  D.C.  generator.  It  is  not 
customary  to  switch  D.C.  ammeters  from  one  ammeter  shunt 
to  another,  first,  on  account  of  the  possible  error  due  to  contact 


From  Generators 

FIG.  100. — Voltmeter  receptacles  for  measuring  voltages  on  all  phases  of 
the  buses  and  on  one  phase  of  each  generator. 

On  high  voltage  circuits  two  voltage  transformers  are  inserted  at  Ti,  and  one 
each  at  Tz  and  Tt. 

resistance,  and  second,  because  each  generator  needs  its  own 
ammeter  continuously  on  the  circuit.  Ammeters  are  used  on 
the  more  important  outgoing  feeders,  but  may  be  omitted  from 
small  feeders  whose  current  is  not  likely  to  be  excessively  high. 

Wattmeters  are  not  ordinarily  used  on  D.C.  circuits,  but  watt- 
hour  meters  may  be  used  on  any  generator  or  feeder  circuit  whose 
total  energy  is  being  observed  closely. 

Meter  Equipment  for  Three-phase  Switchboards. — There  is 
usually  only  one  voltmeter  for  the  entire  board,  with  the  neces- 
sary plug  and  receptacles.  An  arrangement  very  frequently 
used  is  illustrated  in  Fig.  100,  in  which  an  eight-point  receptacle 
is  used  to  measure  voltages  on  all  phases  of  the  buses,  and  in 
addition  one  four-point  receptacle  connects  to  each  generator 
circuit,  to  measure  the  generator  voltage  before  synchronizing. 
Sometimes  an  eight-point  receptacle  is  connected  to  each  genera- 


MEASURING  AND  INDICATING  APPARATUS      163 

tor,  to  measure  voltage  on  all  phases  as  in  Fig.  101.  The  bus 
receptacles  are  then  omitted.  In  still  other  cases  two  voltmeters 
are  used  as  in  Fig.  102;  one  for  making  measurements  on  all 
phases  of  the  buses,  and  the  other  for  one  phase  of  each  generator. 
A  synchronism  indicator  with  all  necessary  plugs  and  recepta- 
cles, should  preferably  be  installed  in  every  large  plant.  Fig. 


/vv 

From  Generators 

FIG.  101. — Voltmeter  receptacles  for  measuring  voltages  on  all  phases  of 

the  generators. 

Voltage  transformers  are  added  on  high  voltage  circuits. 

96  shows  an  arrangement  for  synchronizing  between  two  genera- 
tors, and  Fig.  103  for  synchronizing  between  a  generator  and  the 
buses.  Both  arrangements  are  in  common  use. 

One  ammeter  is  provided  for  each  generator,  usually  with  a 
switching  device  for  measuring  the  current  in  any  particular 


From  Generators 

FIG.  102. — Voltmeter  receptacles  used  with  two  voltmeters — one  indicating 
bus  voltages  and  one,  generator  voltages. 

Voltage  transformers  are  added  on  high-voltage  circuits. 

conductor.  Ammeters  may  be  installed  also  on  the  more  im- 
portant feeder  circuits.  The  connections  may  be  as  in  Fig.  97 
or  98. 

Wattmeters,  watt-hour  meters  and  power-factor  meters  are 
important  in  many  cases;  they  may  be  used  in  any  combination. 


164 


ELECTRICAL  EQUIPMENT 


on  generator  and  feeder  circuits,  if  there  is  special  need  of  them. 
Fig.  104  shows  a  suitable  arrangement  where  all  these  instru- 
ments are  on  a  three-phase  generator  circuit. 

A  constant-current  lighting  circuit  requires  no  meters  except 
an  ammeter  in  the  constant-current  side  of  the  transformer. 
It  may  be  connected  as  in  Fig.  105,  or  the  ammeter  transformer 


Plug 


FIG.  103. — An  arrangement  of  synchronism  indicator  and  lamps  for  syn- 
chronizing a  generator  or  motor  "to  the  buses." 

(shown  dotted)  may  be  omitted,  in  which  case  the  ammeter  is 
put  directly  in  the  lighting  circuit. 

Recording  Meters. — There  are  three  kinds  of  records  made  by 
meters:  (1)  a  graphic  record  made  by  a  pen,  showing  the  fluctua- 
tion of  current,  voltage,  power,  power  factor,  frequency,  or  any 
other  quantity  to  be  measured;  this  is  made  by  a  "graphic"  or 


Current 
Transformers 

FIG.  104. — Watt-hour  meter,  wattmeter  and  power-factor  meter  on  a 
three-phase  circuit.  If  ammeters  are  also  used,  they  may  be  inserted  at  the 
three  dots  at  A. 

" curve-drawing"  meter.  (2)  An  integrated  record,  showing  the 
total  energy  in  watt-hours  or  the  total  ampere-hours  that  have 
been  delivered  in  a  prescribed  time;  this  record  is  made  by  a  watt- 
hour  or  ampere-hour  meter.  (3)  A  record  of  the  maximum 
power  taken  during  any  one  minute,  or  other  interval  of  time; 
it  is  made  by  a  "maximum-demand"  meter.  The  graphic 
meters  may  be  an  unnecessary  luxury  on  ordinary  circuits,  on 


MEASURING  AND  INDICATING  APPARATUS      165 

account  of  the  first  cost  and  paper,  but  in  some  cases  the  commer- 
cial and  industrial  advantages  gained  would  warrant  even  a 
greater  outlay.  Meters  of  the  integrating  type  are  less  expen- 
sive, and  are  in  common  use  on  all  kinds  of  circuits.  Comparing 


FIG.  105. — Connections  for  a  series-lighting  circuit. 

The  connections  for  circuits  B  and  C  are  the  same  as  for  A.     The  ammeter  transformer  is 
sometimes  omitted,  and  the  ammeter  is  connected  directly  in  the  lamp  circuit. 

them  with  the  graphic  meters,  the  graphic  record  has  the  advan- 
tage of  furnishing  information  not  only  at  fixed  times,  but  con- 
tinuously. A  maximum  demand  meter  is  used  where  a  large 
user  obtains  a  low  rate  for  power,  but  is  penalized  for  peak  loads. 


L     CHAPTER  XX 
3      MOTOR  APPLICATIONS1 

In  selecting  a  motor  for  any  particular  service,  the  following 
are  to  be  considered: 

1.  The  kind  of  motor  that  is  best  suited  to  that  service:  whether 
D.C. — shunt,  series  or  compound;  or  A.C. — squirrel-cage  induc- 
tion, phase- wound  induction,  or  synchronous;  also  whether  any 
special  features  (such  as  a  flywheel,  or  series  resistance)  are 
desirable  on  account  of  special  conditions  of  loading,  or  speed 
requirements  (see  Table  XIII,  p.  .167,  also  Chapter  IV,  "D.C. 
Motors,"  p.  22,  and  Chapter  V,  "A.C.  Motors,"  p.  28). 

2.  The  kind  of  system  best  suited  to  that  service:  whether  110, 
220,  440,  550  volts,  or  a  higher  voltage;  and  if  A.C.  whether  25 
or  60  cycles;  one-  two-  or  three-phase  (see  Chapter  III,  p.  16). 

3.  The  size  of  motor  required  for  continuous  duty  (see  Table 
XIV,  p.  170,  and  the  notes  following  the  table). 

4.  The  best  available  speed  of  motor.     See  Table  II,  p.  24,  for 
D.C.  motors,  and  Table  III,  p.  30,  for  induction  motors.     The 
best  speed  for  a  synchronous  motor  is  usually  the  same  as  the 
no-load  speed  of  an  induction  motor  of  the  same  size. 

5.  Changes  in  motor  rating,  on  account  of  (a)  inclosing  the 
motor,  (6)  intermittent  or  variable  loading  of  the  motor,  or  (c) 
effect  of  change  of  speed  on  motor  rating  (see  p.  183). 

6.  Available  sizes  of  motors.     Usually  a  motor  can  be  obtained 
within  25  or  50  per  cent,  of  any  desired  size.     The  exact  sizes 
that  are  available  are  different  in  different  lines  of  machines; 
the  following  sizes  (horsepower)  of  D.C.  and  A.C.  motors  are 
usually  available:  1,  2,  3,  5,  7^,  10,  12^,  15,  20,  25,  35,  50,  60, 
75,  100,  125,  150,  200,  250,  300,  400,  500,  600,  750,  1,000,  1,250, 
1,500,  2,000,  2,500.     If  the  power  required  for  any  motor  appli- 
cation exceeds  a  standard  size  by  5  per  cent,  or  less,  it  is  usually 
safe  to  use  that  standard  size. 

7.  The  total  load  in  case  of  group  drive  is  less  than  the  sum  of 
the  loads  on  the  several  machines,  unless  all  the  machines  in  the 
group  are  operating  at  full-load  at  the  same  time.     Usually  the 
horsepower  of  the  motor  driving  a  group  of  machines  should  be 
40  to  80  per  cent,  of  the  sum  total  of  the  full-load  horsepower 
required  by  the  several  machines  in  the  group. 

^ee  foot  note,  p.  167. 

166 


MOTOR  APPLICATIONS 


167 


TABLE  XIII. — KINDS  OF  MOTORS  FOR  VARIOUS  INDUSTRIAL  MOTOR 
APPLICATIONS  l 


Motor  application 

Motors  usually  preferred 
(See  list  of  abbreviations,  p.  168.) 

Motors  sometimes 
satisfactory 

Machine  shops: 
Bending  and  forming  machines  
Bolt  and  rivet  headers  

Com. 

Shu.,  VS. 

SC.  or  WR.,  Sip. 
SC.  or  WR. 

Boring  mills  

Com. 

SC.  or  WR.,  Sip. 

Drill  presses  

Shu.,  VS. 

SC.  or  WR. 

Drills,  radial  

Shu.,  VS. 

SC.  or  WR. 

Emery  wheels  

GD. 

SC. 

GD 

SC. 

Grind  stones 

GD 

SC. 

Hammers       .... 

Com 

SC.  or  WR.,  Sip. 

Lathes,  axle  

Shu  ,  VS. 

SC.  or  WR. 

Lathes,  engine  

Shu.,  VS. 

SC.  or  WR. 

Lathes,  wheel  

Shu.,  VS. 

SC.  or  WR. 

Milling  machines  

Shu.,  VS. 

SC.  or  WR. 

Pipe  threading  and  cutting-off  machines 
Planers 

Shu.,  VS. 

SC.  or  WR. 
SC. 

Polishing  and  buffing  
Presses,  hydrostatic  

Shu.  or  SC. 
Shu.  or  SC. 

Punch  presses  

Com.,  FW. 

SC.  or  WR.,  Sip., 

Rolls,  bending  .                                      .  . 

Com 

FW. 
WR. 

Saws  .  .   . 

Shu  ,  VS 

SC.  or  WR. 

Screw  machines,  automatic: 
Large 

Shu  ,  VS. 

SC.  or  WR. 

Small       

GD. 

Shapers  (if  not  GD  )    . 

Shu. 

SC.  or  WR. 

Shears  

Com  ,  FW. 

SC.  or  WR.,  Sip., 

Slotters  and  key-seaters  (if  not  GD.)  .  . 

Shu.,  VS. 

FW. 

SC.  or  WR. 

Wood  shops: 
Wood-working  machinery: 
Small  starting  torque  

SC. 

Shu.  enclosed. 

Large  starting  torque  

WR. 

Shu.  enclosed. 

Various  industrial  applications: 
Air  compressors: 
Reciprocating    

Syn  ,FW,  or  Com 

SC   or  others. 

Centrifugal         

WR  or  Shu 

SC. 

Blowers       ....    

Shu  ,  SC   or  WR            » 

Cement  mills: 
Applications  requiring  large    starting 
torque  

WR. 

Applications  requiring  variable  speed. 
All  others  

WR. 
SC. 

Coal  and  ore  handling  

Ser. 

WR. 

Coal  crushers     

Com.  enclosed  or  WR. 

SC. 

Cranes  

Ser. 

WR. 

Elevators  

Spe.,  Com.  or  WR. 

RI.  or  SC. 

1  See  also  the  following,  regarding  industrial  motor  applications : 
G.    Chapter   XVII,    XVIII,  XXXVII,  XL,  paragraph  364. 
S.  Section  15. 
A.  pp.  892,  972. 


168 


ELECTRICAL  EQUIPMENT 


TABLE  XIII. — KINDS  OF  MOTORS  FOR  VARIOUS  INDUSTRIAL  MOTOR 
APPLICATIONS. — Concluded 


Motor  application 

Motors  usually  preferred 
(See  list  of  abbreviations  below.) 

Motors  sometimes 
satisfactory 

Fans: 
Centrifugal 

Shu    SC  or  WR 

Propeller 

Ser    SC   or  WR 

Shu 

Hoists  

WR 

Ser  or  Com  orllg 

Locomotives 

Ser 

WR  if  polyphase 

Paper  and  pulp  mills: 
Small  units  
Large  units  
Low  starting  torque  
Powder  mills  
Pumps: 
Centrifugal  
Reciprocating  

Refrigerating: 
Ammonia  compressors 

SC. 
WR. 
Syn. 
SC. 

Shu.,  SC.,  WR. 
(Motor  depends  on  conditions) 
Com.,  Shu.,  SC.,  WR.,  or  Syn. 

WR    MS 

Syn. 
Shu. 

Steel  rolling  mills  

WR.,  SS.,  FW. 

Com.,  FW. 

Telpherage 

Ser 

WR. 

Textile  mills 

SC   or  Syn 

Shu.  or  Dif. 

Turn  tables  and  transfer  tables  

Ser. 

WR. 

Abbreviations  (arranged  alphabetically} 

Com.  =  D.C.  compound  motor. 
Dif.  =  D.C.  differential  compound  motor. 
FW.  indicates  that  a  flywheel  should  preferably  be  mounted  on  the  motor 

shaft  or  geared  to  the  motor,  to  relieve  the  motor  of  short-time 

overloads. 

GD.  —  Group  drive  by  any  approximately  constant-speed  motor. 
Ilg.  =  Ilgner  system,  consisting  of  an  induction-motor-generator  set 

with  flywheel,  driving  a  shunt  motor.     See  p.  40. 
MS.  =  Multi-speed  induction  motor,  which  has  two  or  three  synchronous 

speeds.     See  A.  p.  977,  S.  7  :  276;  15  : 304. 
Rev.  =  D.C.  motor  specially  adapted  to  reversing. 
RI.  =  Combination  single-phase  repulsion-induction  motor. 
SC.  =  Squirrel-cage  induction  motor. 
Ser.  =  D.C.  series  motor. 
Shu.  =  D.C.  shunt  motor. 
Sip.  indicates  that  squirrel-cage  and  phase-wound  induction  motors  are 

to  have  sufficient  resistance  in  the  rotor  circuit  to  introduce 

about  10  per  cent,  slip  at  full-load. 

Spe.  indicates  that  the  motor  is  to  be  of  special  construction. 
SS.  indicates  that  a  25-cycle  motor  is  desirable,  for  slow  speed. 
Syn.  =  Synchronous  motor  with  self-starting  winding. 
VS.  indicates  that  the  motor  specified  is  especially  preferable  only  where 

variable  speed  by  hand  regulation  is  required. 
WR.  =  Induction  motor  with  wound  rotor  and  slip-rings  connecting  to  an 

external  rheostat. 


MOTOR  APPLICATIONS  169 


SIZES  OF  MOTORS 

In  the  following  table  some  of  the  formulas  for  motor  horsepower  have 
a  purely  theoretical  basis  and  others  are  empirical,  being  based  on  experi- 
mental data.1  The  power  required  depends  so  much  on  the  nature  of  the 
work  to  be  done  that  in  some  cases  the  formulas  cannot  be  more  than  a  first 
approximation . 

Where  the  power  can  be  expressed  in  terms  of  a  single  variable,  that 
variable  is  written  as  A  in  the  formula  in  the  second  column;  the  third 
column  states  what  is  meant  by  A,  or  by  the  several  variables;  and  the  fourth 
column  gives  the  range  through  which  the  data  indicate  that  the  formula  is 
correct.  In  most  cases  the  range  may  be  continued  both  upward  and 
downward,  without  excessive  errors.  Where  this  column  is  left  blank,  the 
formula  holds  for  all  ordinary  ranges. 

1  Most  of  the  formulas  are  revised  from  data  appearing  in  Section  15 
of  the  "Standard  Handbook  for  Electrical  Engineers,"  Fourth  Edition, 
1915  (New  York,  McGraw-Hill  Book  Co.,  Inc.);  and  from  Leaflets  3,516A 
and  3, 554 A  on  "Machine  Tool  Applications,"  issued  by  the  Westinghouse 
Electric  and  Manufacturing  Co. 


170 


ELECTRICAL  EQUIPMENT 


M 

i~ 

>  ^? 

[fl    m    *  /-*\ 

o   c 

£  a  S  « 

S  o                                   o                     •*« 

ij 

Jj]j 

fll    Us            | 

0                          0                   O     O 

•sfli 

co  «-•            «^  <N                eo                     j£ 

•£                        CO                  O   OS 

s°fls 

s  1 

gJS®* 

£ 

tf43 

<N 

>>  *O   CO 

1 

1  *  *f 

a 

1  §* 

1 

a|  | 

d 

ill 

<U/S> 

5  «  •§ 

li 

M  a  ^ 

2.S 

1 

ill9 

^j 

rC 

"3  •-  ^  s 

£  ° 

**                                                                                                                  fl 

fl  oo  "a!    g 

•2^3 

_W                                                                       'rt 

1  0  1  JO 

X.                                        "^ 

M    °    <B    "* 

~  «s 

r 

S               g                  ^3 
^       ^  to               S                   1  iS 

|3"lj    ^                , 

o 

T) 

"o    -3  .2          .*            -a  ° 

J3         ^3    S                     3                             M    S 

1  *1  «  7   1               3 

§ 

*o       *o  "°                 —  «                      *o  "S 

flg«M«g                *;           *oSj 

**  s  i      i        fe  i 

"fi  «  M  ^    H          «s       fe  ** 

£ 

0) 

3     a    ll          °             I"3 

*  M  I-  S  i          -S 

i. 

J£      Q      S  S               S                    S  Q 

11   S  |  J  o  .2               g          .5  S 
&,pc,-^Q                  <j             Qpn 

1 

8, 

0 

- 

\  00   00 

£ 

^           "^           rjj    CO                          CO           •*                     |2. 

^           CO           CO                  Jo      1       I 

^ 

°           ^          °°^"          ^          ^          ^«             •     "^  S 

r-t            ^j            ^H             >    *-*    -^    ^J 

g 

0                                                                                         « 

<N                    '     1   ^ 

"o 

S 

:      :  g1  :   :  5   :  S  i  ^  •  g   : 

°     Z      S  :  ^ 

•3           M          08           M 

.    g)     .     .  ^5     .    «g     .    g     .  .2     • 

111 

8      :>2   :   :^   :  *   :  s   :  1   : 

g 

i-  I-   ^   ^ 

1 

•   3  a      f'.^  ':•'  :.  4  :  1  :  f  f 

«—  •                         '"T         *d          (J3 

i    1    !    1    1 

1 

S  1  1  J  1  '*  If  1  »  :  9  :  -g  I 

51  liiil!!:!;!!  1 

fl  Illil  1  1  H|1 
II  flffijiiffiiit] 

15   fl       •—  —    S  'C  'S   fe  'C    te  'C   fe  3  -g   S  3 
,2«        ooBooo^o^t<<DS*fc> 

S  W        MM        MM        M        M        Q            Q 

i    B    § 

1    I    I    I    1 

•g  o5  -g      3      2      S 

s|a     §     I     || 

I  iiiilil2 

i.ii  >1  '1*5  i 

Q       Q       Q       Q       Q  O 

MOTOR  APPLICATIONS 


171 


IJIf 

0.2  *  E 

0  0 

§ 

cj        fr>_S 

0          00                 00 

1-4                  C$    ^     O 

fi  ^ 

22         22 

22           f        2222 

CO          O                 »O   iO 

TH    (N                                                   «M    >vi     ^1 

^2      •*    $  SMI  **! 

|j£« 

K 

1 

.2 

1 

•^liii  i 

^ 

liMS^ 

j 

42 

sfl^lS 

.1 

• 

illM- 

,nd  other  letters  indicate  (in 
stated  otherwise) 

i  wheels. 
f  wheeJ. 

lammer  head  in  pounds. 
lammer  head  in  pounds. 
of  rotation  in  feet  per  minute, 
ifference  between  radius  before 

L  inches, 
distance  along  axis)  in  inches  ; 
on  the  material  being  machined 
With  a  round  nosed  tool,  for 
;  for  wrought  iron  or  machin 
•d  steel  (0.5  per  cent,  carbon  an 
for  brass  and  similar  alloys,  K 

;  •'    -  1 
i  ^ 

2 
1 

S3       &            °°1:r 
||  f  f 

3        Q             f  |  co  0 

S5fi*Js 

®  5  fl   o  o  t^ 
•S-2  a2  ^|  2 

a  i-S-sS^a 

&,    fc«J 

5     "s  §  |  a 

MM                 "3            ^  5  "•  "g 

.go             ^        ^  .M  3    . 

II         *      ?WH! 

0) 

i 

i2 

^              % 

o 

CO  <* 

io»o       o       ^^      20^ 

1 

$$    ^j    S*8g 

1  '  °  'o  v.  J.  **  "  i  7  ^ 

o 

10       oo  •*>  1-1  j*< 

J* 

|rf 

^    °    *s^3 

33.»^s-;?||i 

2 

^  ^                    ^ 

1 

o 

g 

IS           1 

I 

•3      S    >  3        '•        • 



a    -a    -a            : 

rt             rl 

1 

g     a     «            : 

ft     a     a           "i 

^        'S        *3               a3  2 

:           I!    |      : 

HJ                 fl                 fl                           rt       <r) 

8 

'.    •B-'B       11 

co                                    ca  *Q 

.  .     'S       :  — 
:             >  >  x  TJ  -a    :  2 

a 

15°           ^  S 

03 

S 

'o 

!  stii  II 

Ullr  I! 
elisf4ii^ 

1^l2:l2ra  II 

^lil  lii  jiil 

flO     .     ."S'cB'a5J322lS 

.rt'rtqjgjaJOOjWOOO 

g  iilfl1!  a  I  a  a 

8"  8"  8"  8"  S  8"  8"  a  a  a  a 
4  S  4  a  ^  4  4  3  a  3  a 

•c  °  *c  ®  'C  »  S  *  -g 
O      O      O      B  B  J 

JS^^J^^iiJiiii 

172 


ELECTRICAL  EQUIPMENT 


ange  of  da 
he  formu 
(in  inche 
stated  ot 


o       o  1  <*       ^iX  o  S 

22§  a  osxr;g?38x   rw- 

3  3S;|"  X£X333       .  xx  3 

u   £6  .1  *#>  rn   A  .„ 


>-"   CO 
II 


a!|5 


5 

^  CO  ft}  CQ  £-3 

fQ  tti  ^J7 7  3 

I     >n 


Sc^ 


^    °° 

d  d 


MOTOR  APPLICATIONS 


173 


•gaj? 

°*§i 
Ifii 

sl-S 

Jsa< 


s-s 

*l 


II 

ii 

D    II 


o  o 


ij 

1. 


tl  T3 

^g§ 

?9ff 

•5  ••-  •« 

11.1 

a:  es  ti 


174 


ELECTRICAL  EQUIPMENT 


X 

IO 

y*i 

S 

X 

2t 

5*1 

l^s^ 

•rt  3  J  O 

CO 
CO 

3 

oo    o    a 

•^          CO           O 

3       o      £ 

•2       ^       -*       oo       o* 
X           ««;£      ?|| 

00 

0 

N 

CO 
O 

?|-i-8 

»°al 

(N 
H 

•*        OS         ^ 

N          1 

1      is*    s    8    g 

0 

?0 

Hi- 

6 

2         £  S 

J 

.2  a 

5 

.     s 

£  a 

§ 

•3  'H 

O     o! 

o 

0 

f  J 

.s 

al 

5- 

ij 

|| 

.1 

•11 

pi 

'w  «  .-*; 

0 

i° 

1  °| 

s^ 

S'v 

*°  1  "3 

§  es 

I1 

"o 

. 

llf 

S  ^ 

4J 

a  a  » 

^  3  .u 

'O 

**  s  a     a     s     a 

4A 

-     -g  oj  3 

§ 

p 

a  a  'a     §     |     | 

'QJ 

§'S-i 

0) 

1 

Diameter. 

Si     £ 

I  ! 
«9  l_ 

1  1  1  i   •§   1 

a  a  1    -3    -s    <s 
n  u  -g     g     g     § 

,  .      ^!*S       S     .£       £ 

"o 
"d 

K 

|f| 

|     ^ 
02 

S3 

i 

s~^          C^           C3 

a 

•o" 

IO               TTJ 

CN              I-I              l-H 

M 

5T 

| 

1                 VP5 

jo      cq            ,11 

,_, 

^§ 

^               +              § 

£  o       -f  ^       ^       5,      ^ 

CO  iO    O 

1 

«o 

JO          ^ 

w       ^W       «       8       8 

s 

<             co 

0 

. 

o      o      o 

o 

0 

0 

•  ^ 

:  S  :  «  : 

:8 

•   o 

:  .3  :  ft  : 

:          : 

r  application 

1^ 

03     > 

-Q   S 

•   o     •  oo     • 

.    M  ^   O      . 

11  a  ; 
ill 

§  :•  a  s  •  §  ;  a  ;  a  ; 

4    ,?l  M  1 

S     '  S  °  a   ai    . 

belt  sanders 
idle  
indie  

'    «)     ^ 

^ 
"o 

Circular  saws,  rip, 
Circular  saws,  resi 

per  min  
Planers,  matchers  a 
(feed  40  to  80  ft 
Timber  sizers,  4  h 
min.,  average..  . 

•5    1  B  1  1  1    i    1 

-*   :-§«-S-§     .S     .9 

ff  b  1  1  ?  1  1    -S    1 
1  1    1  1  *  1     a     a 

'S  •**.  s  «  2  -s  »  i  «•  s  « 

|ilfjli|ll*j 

a  »  J  -F*  J  J  *  1  *.  a  1 

H      HO      SM      w      m-S 

Sanding  machines, 
Shapers,  single-spi] 
Shapers,  double-sp 
Lathes,  speed  

Lathes,  pattern.  .  . 
Cranes,  Hoists  anc 
Hoisting,  continue 

MOTOR  APPLICATIONS 


175 


'd 


o 

5^ 


1 


g  «   g 


•s   ° 


11 
ll 


w 

I 

I 

i-S*8 


lafai* 

S  *  .a  &  a  5 

1 J  4S  f  £  I 


1 

ii 


s 


.2  g  -   „    g  - 

•*o&*-sg 

'  §  1  t5  1  2  8 

-  i «  EI  i 


.a -s 
+» ^ 


TJ  13 
0  'C 
oS  ,0 

II 


S.2 

•2  0 


•s  5  a 
0  S  S 


Eniciency 
about  80 


•*  ^^.2.2  i^ 

s?  a  is  ^  "  ° 
•5  -s  a  «  «  ^  £ 

1 1  in  §  I 

H  IN  JB  «  S  9  P 

II  il     II    8    II 


a 

H 

t  ^3 


o 


. 
-S    .2 

1! 

0    8 

.2     ao 


+  i 


T 


i      '•« 


-i  ^ 


in  ii 


iii 


g        3 


a  5 


I!! 
*«« 

T3 


roqoot>         tl  ,3 

a  Z  S  <D  *  5? 


M 


1 


3     § 

8- 


Illlll 

H       H       PP 


^  1  1 1 1  |  J 

" a "  -S  &  i  I 


«      B      B 


5^-2 


•afl 

H? 
i!^ 

ilg 


O        0 


111 

^11 


176 


ELECTRICAL  EQUIPMENT 


MOTOR  APPLICATIONS 


177 


Range  of  data  o 
the  formula  is 
(in  inches  i 
stated  other 


o 

a  . 


j 
!*! 


0.0^3 


^       a 
1    |.a    1 


| 

a  g,'h  c     a 

?§a3       I 


.  2®  .£~       -^«-£^g 

l?i*l 


ats 
illet 
ates 

s  of 


-9  ii  ii 


ii 


5^«3  s 
•P  ~  o  o  a 

Ii 


ill" 

>  -0  .S  -a 


1|| 

Sg  be 

g.y 


1 
i 
ii 


12 


178  ELECTRICAL  EQUIPMENT 

Notes  on  Table  XIV 

The  following  examples  will  illustrate  the  use  of  the  formulas: 

Bending  and  Forming  Machines. — A  machine  taking  work  34  in.  wide 
requires  0.4  (34  —  15)  or  7.6  hp.  From  the  list  of  available  sizes  on  page 
166,  a  7^-hp.  motor  would  be  selected. 

Drilling  Machines. — There  are  seven  different  formulas  for  the  horse- 
power required  for  a  drilling  machine.  For  a  machine  that  is  required  to 
perform  well-defined  operations  the  first  formula  may  be  used.  If  it  has 
two  spindles,  and  is  to  drill  holes  ^  in.  in  diameter  in  brass,  with  a  feed  of 
1  in.  per  min.,  the  power  required  is  2  X  0.3  (or  0.5)  X  0.52  X  1  =  0.167 
hp.  Probably  a  1-hp.  motor  would  be  used  unless  this  is  one  of  a  group  of 
machines  driven  by  a  single  motor  (see  page  166,  paragraph  7). 

If  the  same  drilling  machine  is  for  general  use,  the  power  required  per 
spindle  may  be  obtained  from  the  formula  A/8,  where  A  is  the  diameter 
of  the  largest  drill  that  will  be  used.  The  total  power,  then,  is  2  X  K/8 
or  0.125  hp. 

If  the  diameter  of  the  spindle,  or  the  diameter  of  the  table  is  given,  or  if 
the  length  of  arm  of  a  radial  drilling  machine  is  given,  ordinary  values 
of  power  required  for  such  machines  are  similarly  obtained.  Where  data 
are  obtainable  for  applying  more  than  one  formula,  the  values  of  horsepower 
may  be  computed  by  the  several  formulas,  and  averaged. 

Edgers  (for  Wood-working). — If  an  edger  has  four  saws,  and  is  used  on 
6-in.  stock,  the  motor  should  be  of  8  X  4  X  6  or  192  hp.  A  200-hp.  motor 
would  be  selected.  , 

Ripping  and  resawing  by  band  saws.  The  power  required  for  this  and 
other  wood-working  depends  on  the  dryness  and  kind  of  wood.  For 
ripping  12-in.  (1-ft.)  wet  oak  with  a  feed  of  30  ft.  per  min.,  the  power  required 
is  1.2  X  1  X  30  =  36  hp.  A  35-hp.  motor  would  be  used  for  this  purpose. 

Cranes  ASSUME  THE  FOLLOWING  FULL-LOAD  DATA: 

Capacity  of  crane  (load) 60  tons 

(Weight  of  hook  is  negligible) 

Weight  of  trolley,  including  motor 25  tons 

Weight  of  bridge 50  tons 

Hoisting  and  lowering  speed 20  ft.  per  min. 

Maximum  trolley-travel  speed 100  ft.  per  min. 

Maximum  bridge-travel  speed 250  ft.  per  min. 

Average  trolley  acceleration  and  retardation  3  ft.  per  sec.  per  sec. 

Average  bridge  acceleration 0.5  ft.  per  sec.  per  sec. 

Bridge  retardation  must  be  at  least  as  much 

as  its  average  acceleration. 

Wind  velocity 20  miles  per  hr. 

Area  facing  the  wind 280  sq.  ft. 

Diameter  of  trolley-motor  armature 18  in. 

(radius  of  gyration  =  0.7  X  18  in.) 


MOTOR  APPLICATIONS  179 

Diameter  of  trolley  track  wheel 12  in. 

motor  r.p.m. 

Trolley  gear  ratio, — - — > 2:1 

track  wheel  r.p.m. 

Weight  of  motor  armature  and  gear 1,000  Ib. 

Motor  efficiency  for  each  motor 0.90 

Maximum  hoisting  distance 20  ft. 

Maximum  distance  of  trolley  travel  in  each 

direction , 40  ft. 

Maximum  distance  of  bridge  travel  in  each 

direction 400  ft. 

One-half  of  the  hoisting  and  lowering  may  be 

performed  during  bridge  and  trolley  travel. 
Other  data  are  found  in  the  table. 

DYNAMIC  BRAKING. — As  is  customary,  the  motors  are  provided  with 
"dynamic  braking" — that  is,  in  place  of  a  friction  brake,  each  motor  is 
connected  to  operate  as  a  generator  sending  power  back  into  the  line. 

Let  Pm  =  mechanical  power  developed,  which  is  delivered  either  from 

the  motor  to  the  crane,  or  from  the  crane  to  the  motor; 
Pi  =  electric  power  input  when  the  machine  is  running  as  a  motor ; 
P0  =  electric   power   output    when  the  machine  is  running  as  a 

generator; 

e  =  efficiency  of  conversion  in  either  direction.  It  takes  account 
of  all  or  nearly  all  losses  in  the  motor;  it  also  takes  account 
of  losses  in  gears  and  bearings  of  the  crane,  if  they  have  not 
been  included  elsewhere. 

The  relations  between  mechanical  and  electrical  power  are: 

Pi  =  Pm/e;         Po  =  Pme 

so  that  if  the  same  mechanical  power  is  developed  in  the  two  directions, 

Po  =  Pie*. 

That  is,  the  electric  power  output  in  lowering  a  hoist  or  retarding  a  trolley 
or  bridge  is  ez  times  the  electric  power  input  to  produce  the  same  mechanical 
power  in  the  reverse  direction.  Heating  of  the  armature  depends  on  the 
electric,  not  the  mechanical  power  developed,  so  that  the  ability  to  receive 
is  greater  than  that  to  deliver  mechanical  power.  For  example,  if  a  motor 
has  an  efficiency  of  0.90,  and  is  receiving  123  mechanical  hp.,  the  heating 
is  the  same1  as  if  it  were  delivering  123  X0.902  or  100  hp.  Stated  differently — 
if  a  machine  is  operating  as  a  generator,  receiving  a  certain  amount  of 
mechanical  power,  the  equivalent  mechanical  power  output  (in  its  effect  in 
heating  the  motor)  is  the  mechanical  power  received  multiplied  by  the  square 
of  the  efficiency; 

Pn  -  Pme\ 

This  is  an  important  point  that  is  not  always  understood  clearly. 

1  Subject  to  slight  variations  on  account  of  different  field  excitation  during 
braking. 


180  ELECTRICAL  EQUIPMENT 

COMPUTATIONS. — 

Hoisting  motor: 

20  b^  fiO  ^  2  000 
Power  for  hoisting    =     33  QQQ  x  0*80     =  91  hp*  where  0<8°  is  tne 

efficiency  of  gears  and  crane  bearings. 

Mechanical  power  developed  in  lowering  is  the  same  as  in  hoisting. 
Overall  efficiency  is  0.80  X  0.90  or  0.72.     The  equivalent  power,  in  its 
effect  on  the  motor  during  dynamic  braking  =  91  X  0.722  =  47  hp. 
Trolley  motor: 

.       100  X  (60  +  25)  X  2,000  X  0.015 
Power  for  steady  travel  =  -  — 33  0ftQ  -   =  7.7  hp. 

Average  power  during  acceleration1  = 

100  X  (60  +  25)  X  2,000  /  3\  , 

-GpOQ-          -  i0'015  +  32^1  =  28  hp' 

Average  mechanical  power  during  retardation  is  due  to  the  difference, 

instead  of  the  sum  of  the  effects  of  acceleration  and  friction: 

100  X  (60  + 25)  X  2,000  /  [3 

66,000  -\32 

The  equivalent  power,  in  its  effect  on  the  motor  during   dynamic 
breaking,   is   20.2/e2.    The   efficiency,   e,  takes  account  of  only 
motor  losses,  since  other  losses  are  included  in  the  allowance  for 
friction.     The  equivalent  power  =  20.2  X  902  =  16.3  hp. 
Bridge  motor: 

In  computing  wind  pressure,  the  velocity  of  the  bridge  in  miles  per 

,     250  X  60       _     ..  , 

hour  is  required :       g  ^Q     =  *>  miles  per  hr. 

Wind  pressure  per  square  foot,  w  =  0.004  (3  +  20) 2  =  2.1  Ib. 
Total  power  for  steady  travel  against  the  wind  is 

250  (60  +  25  +  50)  X  2,000  X  0.02  +  2.1  X  280 

33,000  45.5  hp. 

Average  power  for  acceleration  and  retardation  is  computed  the  same 
as  for  the  trolley  motor.  During  retardation,  friction  exerts  a 
retarding  force  of  WF  Ib.  on  a  mass  W,  thereby  producing  a  retarda- 
tion of  WFg/W,  or  Fg.  The  retardation  due  to  friction  is,  there- 
fore, Fg  =  0.02  X  32.2  =  0.644  ft.  per  sec.  per  sec.  Since  this  is 
greater  than  the  required  retardation  (0.5),  dynamic  braking  is 
unnecessary  except  for  emergencies. 

Time  computations: 

Time  for  hoisting  =  20/20 1  min. 

Time  for  lowering 1  min. 

1  f\f\ 

Time  for  accelerating  trolley  =  V/A  =6Q  X3 0.56  sec. 

1  The  effect  of  acceleration  of  rotation  of  the  motor  armature  is  neglected 
in  this  computation.  If  it  is  taken  into  account,  the  following  must  be 
added  to  the  mechanical  horsepower  during  acceleration  and  retardation: 

100  X  1,000  X  3  /  18 

66,000  X  32.2    \    X  °'7  X 


MOTOR  APPLICATIONS 


181 


Distance  covered  during  acceleration 

Distance  covered  during  retardation 
Remaining  distance  of  trolley  travel 
Time  for  steady  travel 

Time  for  accelerating  bridge 
Space  for  accelerating 
Time  for  retarding 

Space  for  retarding 
Space  for  steady  travel 
Time  for  steady  travel 
Total  time  for  crane  travel 


100 
2X60 


X0.56 


=  40  -  2  X  0.46 
=  39/100 
250 

~  60  X  0.5 
_  250  X  8.3 
2  X60 
250 


"  60  X  0.644 
_  250  X  6.5 

2X60 
=  400  -  30 

370 
~  250 

8.3  +  6.5 


=  1.48  + 


60 


=  0.46  ft. 

=  0.46ft. 
=  39ft. 
=  0.39  min. 

=  8.3  sec. 

=  17ft. 
=  6.5  sec. 

=  13.5  ft. 
=  370  ft. 
=  1.48  min. 
=  1      min. 


Since  one-half  of  the  lowering  and  hoisting  can  be  during  bridge  travel, 
and  trolley  travel  is  during  bridge  travel,  the  total  time  of  one  trip 

=  — s H  1M  =  2%  min.,  approx. 

If  the  crane  carries  no  load  on  the  return  trip,  the  time  for  raising  and 
lowering  the  hook  may  usually  be  neglected;  and  we  may  assume  that  the 
bridge  and  trolley  speed  is  the  same  at  light-load  as  at  full-load.     The  time 
for  the  round  trip  then  =  1%  +  2%  =  4^  min. 
Root-mean-square  computations: 

Root-mean-square  of  power  in  hoisting1  = 

2x4?  +  47'xlk  •  48  h*- 

That  is,  the  motor  should  be  able  to  carry  48  hp.  continuously,  and 
91  hp.  for  a  short  time. 

The  root-mean-square  horsepower  of  the  other  motors  is  found  in  the 
same  way.  Motor  speeds  and  gear  ratios  must  be  determined 
after  referring  to  speed  characteristics  of  available  motors. 

Cement  Plants — Ball  and  Tube  Mills. — These  mills  are  cylindrical  in 
form,  as  in  Fig.  106.  They  are  filled  about  one-half  full  of  cast-iron  balls, 
pebbles,  or  other  means  for  pulverizing  the  cement,  and  they  are  filled  to 
the  same  depth  with  the  material  to  be  pulverized.  A  motor  is  connected 
to  each  mill,  rotating  it  and  causing  the  balls  or  pebbles  to  fall  on  the 
material,  pulverizing  it.  The  center  of  gravity  of  the  contents  is  at  a  dis- 
tance OG  from  the  center  of  rotation;  this  distance  can  be  obtained  from 
Table  XV,  after  finding  the  area  of  cross-section  of  the  material.  The  slope 
of  the  surface  at  which  the  balls  or  pebbles  begin  to  fall  is  about  45°  from  the 

1  Theoretically,  the  values  of  PI  and  P2  should  themselves  be  the  root- 
mean-square  values,  for  times  t\  and  h  respectively.  Practically,  the 
average  value  is  easier  to  find,  and  in  all  ordinary  cases  the  error  is  negligible 
in  assuming  that  the  average  values  are  the  same  as  root-mean-square  values 
for  the  respective  times. 


182 


ELECTRICAL  EQUIPMENT 


horizontal.  The  power  in  foot-pounds  per  minute,  required  to  rotate  the 
mass  is  2?r  X  torque  in  foot-pounds  X  r.p.m.  From  these  data,  having 
given  the  weight  of  material  and  r.p.m.,  the  power  expended  in  the  tube 
can  be  computed.  The  formula  for  horsepower  given  in  Table  XIV  is 
based  on  the  assumption  that  the  angle  is  45°,  and  that  about  25  per  cent, 
is  added  to  the  power  expended  in  the  tube  for  gear  and  bearing  friction. 
These  are  usually  good  assumptions  to  make. 

ASSUME  THE  FOLLOWING  DATA: 

The  mill  contains  60,000  Ib.  of  cast-iron  balls,  and  the  interstices  between 
the  balls  are  filled  with  material  to  be  pulverized.  Cast  iron  (solid)  weighs 
450  Ib.  per  cu.  ft.  Cement  material — if  balls  were  omitted — would  weigh 
85  Ib.  per  cu.  ft.  Of  the  total  space  filled  with  the  mixture,  60  per  cent,  is 
occupied  by  iron  and  40  per  cent,  by  cement  material. 


ast  Iron  Balls 

r  other  means 
for  Pulverizing, 
and  Cement 
Material  that  is 
to  be  Pulverized 

FIG.  106. — Position  of  load  in  a  tube  mill  during  rotation.     The  surface 
inclined  about  45  degrees  from  the  horizontal. 
O  =  Center  of  rotation.     G  =  Center  of  gravity  of  load. 


The  inside  diameter  of  the  tube  is 
The  mill  makes  22  r.p.m. 


ft.),  and  the  length  21  ft. 


COMPUTATIONS: 
Cubic  feet  of  mixture  = 


60,000 
0.60  X  450 


222  cu.  ft. 


TABLE  XV. — CENTER  OF  GRAVITY  OF  SEGMENT  OF  A  CIRCLE 
(Used  in  Computing  Horsepower  of  Tube-mill  and  Ball-mill  Motors) 
Columns  headed  "Area"  give  the  area  of  the,  segment  in  terms  of  the 
square  of  the  diameter,  D2.     Columns  headed  "Distance"  give  the  distance 
from  the  center  of  gravity  to  the   center  of  rotation  in  terms  of  the 
diameter,  D. 


Area 

Distance 

Area 

Distance 

Area 

Distance 

0.05D2 

0.431D 

0.3927D2 

0.212D 

0.6D2 

0.103D 

0.1D2 

0.391D 

(L£  of  com 

plete  circle) 

0.65D2 

0.076D 

0.15Z)2 

0.355D 

0.7Z>2 

0.049D 

0.2D2 

0.322D 

0.4D2 

0.208D 

0.75D2 

0.021D 

0.25D2 

0.292D 

0.45D2 

0.181D 

0.7854D2 

0 

0.3D2 

0.263D 

0.5D2 

0.155D 

(complete 

circle) 

0.35D2 

0.235D 

0.55D2 

0.129D 

MOTOR  APPLICATIONS  183 

222 

Sectional  area  =  -^-  =  10.6  sq.  ft. 

In  terms  of  square  of  diameter,  area  =      '--2    =  0.32D2 

From  Table  XV,  distance  to  center  of  gravity  =  0.252D  =  1.44  ft. 

Weight  of  cement  material  =  £4£~^  X  60,000  =  7,600  Ib. 


Total  weight  inside  the  tube  =  60,000  +  7,600  =  67,600  Ib. 
Power  required  to  operate  22  X  1.44  X  67,600/6,000  =  280  hp. 

CHANGES  OF  MACHINE  RATINGS 

(a)  Enclosed  Motors. — A  motor  designed  to  be  run  open  (with 
windings  exposed  to  the  outside  air)  may  usually  be  run  enclosed 
(with  lids  closed,  decreasing  the  cooling  effect  of  the  outside  air) 
and  carry  continuously  two-thirds  of  the  rated  full-load. 

(6)  Intermittent  and  Variable  Loads. — Electrical  machines, 
transformers,  and  other  apparatus  having  heavy  windings,  which 
require  6  or  8  hr.  to  approach  their  maximum  temperature,  do 
not  usually  heat  the  windings  excessively  when  they  carry: 

150  per  cent,  of  continuous  full-load  rating  for  1  hr. 

200  per  cent,  of  continuous  full-load  rating  for  }/%  hr. 
This  logically  implies  that  the  machine  is  not  already  heated 
when  the  overload  occurs;  if  it  is  overloaded,  following  a  long 
full-load  period,  the  size  of  the  machine  should  be  increased  a 
little  on  that  account. 

If  the  motor  is  loaded  only  a  few  minutes  at  a  time,  or  if  the 
load  fluctuates,  the  average  heating  of  the  armature  may  be 
obtained  as  in  the  case  of  crane  motors.1 

Change  of  Speed. — A  motor  is  sometimes  run  at  a  higher  speed 
than  that  for  which  it  was  originally  designed.  For  this  change 
it  is  necessary  (1)  that  the  principal  field,  or  a  special  commutat- 
ing  field  be  strong  enough  for  commutation  at  the  higher  speed; 
(2)  that  the  mechanical  construction  of  the  armature  be  strong 
enough  to  withstand  the  increased  centrifugal  force;  and  (3) 
that  the  armature  be  well-balanced,  and  the  bearings  ample,  to 
prevent  excessive  heating  and  wear  of  bearings.  Under  these 
conditions,  the  speed  of  a  D.C.  motor  can  frequently  be  increased 
to  twice  its  original  speed,  by  weakening  the  field.  The  increased 
speed  increases  the  ventilation,  and  thereby  reduces  the  heating 
of  the  armature,  so  that  at  double  the  speed  the  motor  will  usu- 

^ee  "  Hoisting,  Trolley  or  Bridge — Intermittent  Duty  and  Variable 
Duty,"  Table  XIV,  p.  175,  and  the  footnote  referring  to  this  application. 


184  ELECTRICAL  EQUIPMENT 

ally  deliver  20  per  cent,  more  power  than  at  the  original  speed 
(see  p.  23  for  allowable  torque  at  higher  speed). 

If  the  commutator  and  winding  will  stand  a  higher  voltage,  the 
speed  may  be  increased  by  increased  armature  voltage,  instead  of 
by  field  control.  In  that  case  the  armature  will  not  heat  excess- 
ively, even  when  the  power  delivered  exceeds  200  per  cent,  of 
full-load;  usually  commutation,  rather  than  heating  of  the  arma- 
ture, limits  the  maximum  possible  load  under  these  conditions. 

When  lower  than  rated  speeds  are  obtained  by  inserting  resist- 
ance in  the  armature  circuit,  the  maximum  allowable  horsepower 
is  reduced  about  in  proportion  to  the  speed,  since  the  armature 
current  is  the  same  at  the  reduced  horsepower  as  at  full  horse- 
power at  full-speed. 


CHAPTER  XXI 
COSTS 

Even  under  the  most  favorable  conditions  it  is  not  possible  to 
give  a  simple  general  expression  for  costs,  that  can  be  applied  to 
all  kinds  and  grades  of  apparatus.  This  is  especially  true  under 
the  present  fluctuating  industrial  and  commercial  conditions. 
The  costs  given  in  the  following  table  should  therefore  be  checked 
with  quotations  from  manufacturers,  before  they  are  used  as  a 
basis  for  installing  equipment. 

For  cost  of  copper  wire,  see  Table  XII,  and  accompanying  notes, 
pp.  81  and  84.  For  cost  of  batteries  see  p.  51. 

Costs  of  engines,  generators  and  motors  depend  so  much  on 
speed  and  other  features  of  design,  that  it  is  impracticable  to 
express  even  a  close  approximation  to  costs  in  any  simple  way. 
The  following  expressions  are  not  put  in  the  table,  because  they 
are  only  rough  approximations;  they  will  be  found  to  be  too  high 
for  average  conditions: 

For  1  to  25  kw.,  or  1  to  35  hp.  D.C.  generators  or  motors  running  at 
1,200  to  400  r.p.m.,  $75  +  $21  per  kw.,  or  $75  +  $16  per  hp. 

For  50  to  2,000  kw.,  or  60  to  2,400  hp.  D.C.  generators  or  motors  running 
at  400  to  200  r.p.m.,  $300  +  $8  per  kw.,  or  $300  +  $6  per  hp. 

For  60-cycle,  100  to  800  kva.  alternators  or  synchronous  motors,  running 
at  300  to  100  r.p.m.,  $1,000  +  $14  per  kva. 

For  25-cycle,  1,000  to  2,000  kva.  alternators  or  synchronous  motors 
running  at  100  r.p.m.,  $7,000  +  $7  per  kva. 

For  25-cycle  alternators  or  synchronous  motors,  add  10  per  cent,  to  the 
cost  for  60  cycles. 

For  60-cycle  induction  motors,  25  to  300  hp.,  $250  +  $5  per  hp. 

For  25-cycle  induction  motors,  25  to  300  hp.,  $200  +  $9  per  hp. 

For  motor  generator  sets,  sum  of  motor  and  generator  cost. 

For  300  to  1,000  kw.  synchronous  converters,  $4,000  +  $15  per  kw. 

For  A.C.  or  D.C.  turbo-generator  sets  (including  turbine),  up  to  300  kw., 
$300  +  $30  per  kw. 

For  60-cycle,  500  to  5,000  kw.  turbo-generator  sets,  $5,000  +  $10  per  kw. 

For  60-cycle  turbo-generator  sets,  above  5,000  kw.,  $30,000  +  $6  per  kw. 

For  reciprocating  compound  steam  engines,  $500  +  $20  per  kw.  capacity 
of  the  generator. 

185 


186 


ELECTRICAL  EQUIPMENT 


oo^ag 

fe*5  P.C8  m 


3*§J 

s!s| 


a  a 

S  S  „ 

c3    cS  ^ 

1C  »O  fl 


4! 

»8 


!! 


II 

•3  > 
>  o 

|| 

33 


t 


O 
•        »o 


B. 


2          IIS 


£  §  £ 
3  |  3 

Us- 
414 


Ij 
It! 


3      3 

2       E 

1      2 


*      I 

I 


a  » 

*o  *» 


a\ 


3     •* 


» s 


II 


1-B 


il 
«1 


a 


Si 


15 

!!?!!!  !?S    1 


COSTS 


187 


«3 

f 


ftft'o 

^ 

'C   1   03   » 

8-og5« 

.0 

d 

1-1 

^  d  §  aa 

TT 

d 

hMli 

1 

^ 

^                              {^ 

ft 

£ 

^  2  tj 

CN 

es 

"1 

.                           lg     .    **»      .    P*                 10          f^ 
g                        (N          *O          CO                 M          «O          ^ 

73  T3 

| 

1 
d 

S 

CO 

CO    CO    (N 

d 

3 

d 

§ 

<N 

O 

° 

o 

+  d       odi-HcoddcioJco 

i-H          i-(O5O<iOCOOO»O(Ni-H(NOOfOTti»O 
rH           CO           •*                  W           CO           »O 

II 

1! 

o  5 

fH 

0 

CO 

°  1 

d   os 

«     ft 

•^ 

H     ft 

a 

0   0 

s  * 

1 

s's 

§ 

§ 

ll 

D 

o 

3  £ 

2 

o 

d   ® 

ft 

ft 

8  ^ 

8 

8 

0  ^ 

"* 

CO 

•* 

1| 

!|W 

1| 

«5i 

rt 

CO 

IN                 i-li-l(N<Ni-li-l(N(N 

tl 

e3    w 

o  d-* 

g   a> 

CQ 

"o  0 

^-> 

f—' 

a 

2              2      2      2          2      2      2 

.2  I 

fsf 

?S4 

1 

9 

S           §88        S     8     8 

ll 

^» 

d 
<1 

^    <N     g 

So* 
go 

5 
2 

1 

>e 

1 

ijf   |i|g|i||i|i|i| 

ss^ 

ft 
03 

M 

Ammeters 

Voltmeters 
Ammeter 
shunts 

Ammeter 

2 
d 

3 

,d 

• 

Ammeters 

>•          •<>-c»pLt-^>'o3cw 

strument  transfo 
ransformers.  Cc 

%. 

1 

;r 

d  •** 

73    3 

l-g|l 

X 

<0 

1  1 

III" 

IN 

Ci' 

5              ^     1 

ll 

J2$ 

T                                                                                                                                                                                             V 

3  -g 

*d 

i 

1 

3 

Illuminated  dial,  for  large 

switchboard  panels,  orswing- 

ing  brackets. 
Negligible  temperature  coeffi- 
cient; negligible  thermo-elec- 

tromotive  force;  thus  avoiding 

errors  due  to  heating. 
Low  price;  only  moderate  ac- 

d 
*o 

0) 

•o 

d 

p 

_S 
OQ 

£* 

§=5 

A.  C.  Switchboard  Meters. 

Low-price,  simple  construction, 

only  moderate  accuracy;  am- 

11         p 

8?                     |§ 

*»  O                                       ^  '13 
30                                          ,>>  "8 
0  T}<                                        73    3 

^  <N                                              §•« 

•Id                  1  .a 

§                  ft.. 

11       ll. 

al                   «Be 

ts  §  a            -asts 

d   <-   ®                             •?•   o   d 

•*»  -                        nj  — 

»  Meters  and  other  apparat 
primary  rating  and  insulation 

188 


ELECTRICAL  EQUIPMENT 


mi 


s 


to  400 

50  to  7 


it 


a     8  a 

•2   2   §  -2   £ 

III!! 
iliill 


-  s  jg  3  -  „ 

q*    o    03    g 
£*    &^ 


J.§ 


•a"  a 
I* 

-s  § 


5  s 


COSTS 


189 


o  *  «  2 

ill! 


3  £3 


O   CO  (N    i-l  (N 

(NO  -10  O 

do  do  d 

CO   ^  (NO  IN 

CO   •*  lO   CO  CO 


8     8    "g 


8 


s  +  s 


§        8 


«S««S«fl      «      «      « 

<i;  r-i  <q  <;  — i  <i;  <q       •<       •<       •<; 


o 

i! 


I! 


11 


si 
f'S 


III 

2  a  | 

o  45   S 


1111! 


II   ill 

a  i  oo  S^o 

{||i|a 

M    ^    «  ^>    ^     Ui 
02     ft         PL| 


i 


XX 


CO    CO 

X  X 


^H    i-H  i-K   >-\ 

iO    *O 


s 


• 


li 

5  .2 


ill 

1!i 

113 


!  I 

?   o 
co    o 


•2  ^  "S 


190 


ELECTRICAL  EQUIPMENT 


1£  S  § 

lll 


s 


o 

4-  8 


8        8 


D,  O 

s  8 

o£ 


58 
88 


.2      -2 


S"»S-»      S"» 
o  fl  o  fl       o  fi 


II 

o    •* 


li^iiiii  I  ! 


fr, 

-si 

|i 

M 


I! 


i!j 

t 


S3 

>-,      d 

O.  113"^ 


11 


XQ   «•; 


lit 

§  2  * 
_  '3  c 

i-H'l 


T3    >>4J 


• 

J?  &  S 


COSTS 


191 


•H"* 

88*51 

is'i 

S"°  1-2  » 

Ifllp 

£ 

8 

^^ 

^ 

j 

(N 

»o  ^ 

fc* 

"*5 

•"1       "3       ^ 

3 

0  0 

d 

w 

M       d       N 

-a 
a 

g 

OS  0 

OS 

OS 

OS          -<          •* 

« 

1-1 

CO 

I-l 

81        ^ft        C4 

§ 
1 

II 

ft  ft 

a  s 

V 

f» 

C8    * 

1 

f  | 

»j*  o" 

W    rH 

o 

«  a 

ft 

ft 

ft   o 

§§ 

6  * 

o 

2  a  fl««  « 

%ii 

OJ   O   CO 

S|-« 

o  t-  a 

111 

i  i  '  1 

ft 

S    n 

|1 

1  " 

jljjjl 

—  a  ** 

5  xo 

O   O 

T*     O 

O   °'    O   °"    O   O 

b  °  ° 

Q          ft      O 

3  oo 

J  ^  J  *  2  ** 

w 

S    03  CO* 

IQ 

a 

1 

A 

i 

1 

III 

03 

I 

2 

2 

2 

222 

i| 

o 

13  M 

a  1 

a  S 

ii 

s  It  ill 

^J     Q, 

ft  ft 

£  a 

2  a 

£  S 

N     8     M     9     M     M 

i 

If 

0 

O  *" 

1| 

5  ^  §  J5  §  ^ 

t/S'S'g 

1 

3 

c 

I 

E 

g 

M 

13 

1 

cc 

For  above  meters;  248  ft.  per 
roll,  for  feeding  at  2,  4  or  8  in. 

per  hr. 
INSTRUMENT  TRANSFORMERS 
Through-type  indoor,  second- 
ary capacity,  25  to  50  volt- 
amp.,  compensated  for  10  or 
25  volt-amp.,  for  slipping  over 
a  cable  stud  or  busbar; 
nominal  secondary  current,  5 

amp. 
Dry-type  indoor,  compact,  low 
priced;  capacity  10  volt-amp.; 
compensated  for  10  volt-amp.  ; 
nominal  secondary  current,  5 

amp. 
Dry-type,  indoor,  capacity  50 
volt-amp.;  compensated  for 
25  volt-amp.;  nominal  second- 

ary  current  5  amp. 
Same  as  the  foregoing,  except 
insulation. 
Same  as  the  foregoing,  except 
insulation. 
Dry-type,  outdoor,  capacity  50 
volt-amp.  ;  compensated  for 
25  volt-amp.;  nominal  sec- 
ondary current  5  amp. 

192 


ELECTRICAL  EQUIPMENT 


Ml 

S 

i 

""I             .          10                      fc>-          S 
<N                       CO                      <*            . 

^                      °° 
S                   ^ 

d 

$   II 

,              0     >» 
-4-           -.    o 

§ 

+      +      o               o      ° 

d                       + 

i    Is 

'oo 

-|-                                              "J 

t. 

d 

s  -               2 

g                       S 

fe     $$ 

d 

1 

i 

'o 

Accuracy; 
maximum 
error  in  per 
cent,  of  full- 
scale  reading 

h-g 

§d      ° 

§ 

la 

S       S 

- 

0  H  9 

83-OW-b^t-                      OS                       * 

§      M 

:l! 

0>0>go3         0           0^ 

si 

l|l|li°    s 

>> 

%> 

d 

fn  d 

iOi-niO(NW5»O                iO          O 

°                  ^ 

^ 

, 

^22                   22 

§                      6 

S       S 

5* 

ill      i  i 

2                    g 

8      8 

2 

^      43      43               £      £ 

2           *H  2  t.  2 

*1 

|!|Sf  i      «|}| 

a  S           1 

3          5  |  -S  1 
«           S  «  d  * 

§       Sail 

a* 

§S|g|o            1  If  1 

o^o^o^        g^^"" 

•30          ^ 

>  ***          > 

s,      |  &!  a 

."a 

111" 

m 

Q,      ft     £•§*!      S      8  ^ 

11     1§£l§     4 

is    &§ 

1 

1 
02 

i  I  Illfi  I! 

M          »          §§,flSO           £g 

.g    .g     c-  a  .  *  a    g  o 
1     1     |8t«*S     S? 

I;l^!fi  a; 

a®           T3     g   "3     W     0           *3   -S 
g^gjr^  :-i 

!JUP8!hlJ 

lljlgslljlll 

200  volt-amp.,  compensa 
for  15  volt-amp.;  nomi 
secondary  voltage,  100. 
Oil-insulated,  especially  sui 
for  voltages  that  are  too  h 
for  dry-type  transforme 
capacity,  compensation  s 
secondary  voltage  same 
above. 
To  correct  the  voltmeter  in 

cation  for  ohmic  and  indu 
ive  line-drop,  on  three-phi 
60-cycle  circuit. 
Same  as  the  foregoing  exc 
for  single-phase  or  two-phs 
For  25-cycle  circuit. 

COSTS 


193 


s-S1"3      ' 

1-olS. 

S  fl  OtJ  M 

jj 

T3  §-^  2  03 

2 

,>8a 

co 

£-13,  *  £ 

CO 

1 

'i 

"o 

§                 +                       8 

8 

TJ 

CO                                        CO                                                       CO 

1C 

fl 

G 

1 

'R 

a           2 

a           a 

S                 ft     . 

—                   -<J 

I 

5     Kflft^- 

§  |fl^1 

°  iist*s« 

3   Bass's 

a           £88 

<                            4* 

L,               M  fl» 

C           o  £  a. 

—  * 

§ 

*c3  d  •*•* 

(N 

5      jtj  > 

Q            3  g  0 

I           I               % 

s| 

c            2 
D            - 

<J                          •<                                   <J 

^          ft 

II                1 

1 

H               ^| 

13°                                              u 

^ 

<- 

<               1a 

1                ll                  11 

(f-l 

3      S 

o                  rt  *                     «• 

0   h 

9 

^             o"l  ^ 

f                 » 

S 

'aj'o  §  fl 

00                                                                                               Q 

x                 x                       2 

X 

00                                     MO                                                   X 

«N 

to 

s; 

•III 

CO                                                                            Tt< 

M 

i 

a 

• 

s 

~c3 

1 

03 

fiiij  pllfll  il-1 

lll||1  Iflif!!  1*1] 

«tJIiiL!j}s!lSjJ!^ 

$^  |l|i|  ill  i^  loil  |i  £s 

tfQO                 ^-^o^   °  o-2-^  °   ftora   ° 

current  is  reversed,  or  operat- 
ing from  2  shunts  in  series  on 
about  25  per  cent,  reversal; 
especially  suited  for  battery 
charging. 
Operate  at  any  desired  vol- 
tage; especially  adapted  to 
battery  charging. 

13 


194 


ELECTRICAL  EQUIPMENT 


S*al 

111  Si 

fe§Jls 

(gJ-ft0^ 

i 
| 

8                                    8 

8 

fc 

0 

00                                                                                    CO 

(N 

"T" 

_|_ 

d 

i-H                                                                                                  TJ< 

(N 

CO 

3 

ci 

0 

E 

5 

o9 

1 

1 

5 

o 

I 

6i  ^i 

o3  d  d  n-i  o5 

3 

E  §  a 

o  co 

PM 

»g- 

0 

JU    <D 

sS  5 

1§| 
""*£ 

! 

P 

>o 

Q 

1 

a  6  M 

Jr 

1                        I 

1 

i 

1 

0 

| 

O 

i 

QQ                                                                                                   FH 

CO 

^-N 

,,^  ^ 

c8  ^ 

fl 

Q? 

a 

J3 

2*"1 

^ 

CO 

hH 

*O  tj 

> 

1                               s  « 

5 

1 

13 

K*^ 

'a  ®* 

1                                1  * 

•a 

M 

1 

1 

a 

§ 

O                                                        P^ 

> 

Q 

« 

H 

!& 

<0 

§1 

f 

X 

sH^" 

X 

S|a 

CO 

iisi|ll*Jf8-ll|I 

bO 
_fl 

o 
,d 

c 

i 

5  4S 
fc  -S 

«  S1 

.2   § 

d 

0 

Special  features  and  use 

•J   i  c^o^ft-a'?-3.3   g  §  i  *   o   2^  ° 
S  >»  '3      a-0§          »  1  *  >>  *  '1      fe  8 

If  Is  H  f^lil  &lf  ^1UII 

'S 

d 
5 

1 
^ 

2 
•3 

33 

high  or  low  voltage. 
Auxiliary  relays. 
Used  in  combination  with 

overload  or  reverse  po 
relay  to  introduce  a  defii 

time  element  in  the  operat: 
For  ringing  an  alarm  when  ; 
particular  abnormal  condil 

occurs  in  any  circuit. 

COSTS 


195 


lllfi 

ilin 


^      3 

CO 


®    .  *  ft 

o    a|ag 


. 

8,  *  .s 


, 
>> 

" 


Full  scale  or 
ormal  curren 
or  voltage  « 


^rf      ~-      |-      |« 
>S.°3.o5.o3. 
oftoftofto        ft 

.i-H     S       -CO     oSCOCO     SCOCO     « 
CO  CD  i-H  CO 


Is 


+3       0) 

c  ."a 


02^ 


-2 


a. 


«  X 


«   it 


196 


ELECTRICAL  EQUIPMENT 


aao 

!i|ll 

nil! 

{^ 

g 

^J         ^^^^^ 

_,«?_<_,       •^*^'^T'Jo                               o 
•^.•^•^i>ot»-*'                              o                o 

M^OOO                r-i^(NO                                              •                       "5 

TJ 

+  +  +  ++  +  +  +  +                                             •*                         M'                        "5 

d 

b-^HOOO           OOO>OO 
i-ICO>O<O          C^'^t-'OS'O 

1 

•s 

'ooi'3^'o®'oJ2                                ^a          £  a          ^ 
a-3  a  g       p,  -3  a  o                                 o  g            o  g 
iaia^aia                                >  &           >  * 

'gogS'MgSs                                   o§            o§            o 

f 

"S 

a 

££H£B.HHJ                      3  N       S  M       S 

i 

s 

A 

I 
T| 

"* 

t 

II  :J! 

BO 

g 

ijg<!.i 

2 

1  si  § 

OH 

&a* 

, 

CM 

o  -, 

t£ 

•si* 

s  §        ^  §      ®  «             ^^1 

0 

«|l 

°od           ^°a§^                       o§°a 

1 

IIs 

|«1      gll    S§              s.E-ll 

XVI.—  C( 

Jl 

'3                          '3                    -|    2                                 «         .-S          a         .t3          OQ 
a>                                »    ,     3         £     ,     3         a) 

•8              '®  B           1  "S                      ^  i  1  1  5  |J  'i  J 

B|       «s     -I          l!o|^?«ll 

1  1         1  2      1-1              "  §  '^  8  "  1  -  2  1 

lon-auto- 
xatic  circuit 
reakers) 

3 

M 

u*          $*       ^«               g-s.ng-a.og 

—    C  ^5 

H 

ft, 

P   r!        ""* 

OS'S  •+* 

S|§ 

.2 

8 

g^                     -S°S>°o                         -S^c 

•N 

•fl       iltjll         ll1 

1 

8 

is         H|fl"^        Hi 

§  -2                   -g,i'®-St£!oa                  ^*»^ 

3 

°o                     ^2tel)"                      S"o 

1 

|4q              Ss||g||                 "=3^ 

**" 

sl«         H*.™           If! 

03 

1 

|Is     llfilll      Jlli 

197 


a  i,  o 

•^  c8 

lilii 

<d  OS  ft 

e 

. 

m 

IQ 

M9 

1C   iO   O   O          O          O           Q. 
N    C<   1C   O          *O          "2 

Q 

00 

<N                         <N 

.S 

d 

^ 

"2^^    "    «    5      2 

1 

o 

M 

eo           ^  <L 

d      d      d      §      ^ 

1 

-2 

Jj 
I 

:°     si 

s         S-f 

^  ^  j?  a     s     e           £ 

ft&a*    «   2    3    0 

NM-4<»O          »C          »O           ^3           "5 

05 
S 

•  FH 

ft 

a        fj^ 

^  |   &   o        o        o 

222>o     «     «     ^4.^-5 

1 

ft 
ft 

•5J 

|| 

>>    >l           >>          •£! 

g-o       "g,  g  S 

?•!    3  ,-| 

g  a     gra« 

S  S  S    rf          a          n          ^o^O 

5i5^'l^^3^5w3'S 
^^^^^^  a^  a^  a^| 

alSSjo<NJ2cojc^3o3^« 

|| 

'.- 

S|® 

5  S     ^08 

-2   g  c? 

'o          2  <j  rt 

Ml 

•      |3    • 

--    S    M 

to  2*  §  —  >       S1 

*"3  o  ^ 

<i    §   "1    0   1C    03 

fa  fl 

•*                  (N 

i 

°l 

1 

Oil  switches 
(non-automatic 

|1ijli| 

i?  Hi  il 

m-ii 
Igllsil 

rsTsii 

jiiiii. 

liH.i  - 

rs   a   £   s  a  o  ^2 
O  ^'S  «3 

•-'".2 

d      *» 

a    I 

^  '*Tl 

e9           > 

t/g*rt  OB 

g 

m          rl    O 

|=i! 

O 

i    ^ 

c  § 

S      3  £ 

J£ 

fa       fa  '8 

ft 

o 

1     «"  i   1 

^3'i2      S  «  >r 

S 

s 

s       Si    1 

«<H     ^^    •  «        3                            '«                ^ 

s 

0) 

S           *>  "3 

0  0   "g  ^2      _          g    MQ 

o 

3 

M                   ><     ®           "fl 

o         -«3      § 

i 

•3 

'3           If  2      ~- 

^        Sow        g   ^  c*i 

1 
1 

i 

i 

s       |-§    1 

«          «2  "3       ° 

5           2  9"      S 

•£  4<       o 

o2-s^|     ~  £  s- 

-rt  o  "2  .£    o         oT         * 

"lai  lift 

«  ^«3  S      S  g  g£ 

•| 

72 

if 

li  ss  i 

g  ft-      g    *      ^ 

§  *  a  t*  *       -  5  ^  "? 

«  a|  &s    ^It-a 

v   v    S  S3    fl          p,  -+-»  *jj    bo 

gg|sa     Bill 

OQ  ^ 

w  ^      «n  w      fe 

e^ftSB     ffi  -  a  « 

198 


ELECTRICAL  EQUIPMENT 


fa-3ft' 


CO  ^Jl  lO  1> 

odd  d 


»      ^»        »        «  TO         *  t-H 

T*     <N     t-     i-H     TH     »H  g 


III   -all 


0  amp. 

o  50  kva. 
to  200  k 


as' 


08  > 

K 


o  o     . 

o   ^  •*»   ft 

•i  n  o  •  § 

CO     O  i-l     C3 


i! 

™r  ft 


« 

co 

-s| 

•la 


a  f 


a  f 


Ml 

•i«' 


S     O     c3    i1 

^  a  ts  -s 


§•1 


COSTS 


199 


ftftO 

Tt< 

""*  o3  tf>  <» 

5-3  !l« 

2  c.2g  M 

03 

s  ss    s    * 

01                                         £i. 

5;    i 

2 

i-<    CD    00    »O                      O    "3             rH 

;j      2    i 

4-         4. 

S-       o 
o       o       o 

s    S    +    + 

.2 

So  o  o            o  o       o 
co  co  co             co  10        o 

§0 
CO                 00          »O 

CO          CN 

4j 

i—  *    CN 

O 

O 

1 

"rt 

6 

I 

o  o  o  o            o  o       o 

*O    (N    >O    r-l                  »O   i-H           *O 
O     O     0     O                   O     0            O 

^1        ;1     1 

-2°          ft      ? 

•i     -S     -2 

ft    B.    a   2 

M 

< 

rH    IO    TH    O                      O    O             O 

«s.    1  1 

£     £     S     p1 

p.rt'^    O    (H 

•3=1 

O           O                        <n               -TO 

-2-2               33 

.2               3 

1   8   * 

i~f 

3       "o                  >             > 

>           >                       0                0 

1          1  1 

1        o        £ 

o  B  "**  -2  o 

If 

^  js  ^  j§            co            o 

1      l« 

!*l*|* 

& 

S       2               2           2 

s      -s   4 

S       S       k 

jn 

"-I-2 

II          II 

1               ft      ft 

I* 

i 

^O            O                            O                    O 

98                §         «  a 

03            c4                          o3                   C8 

H       H                 H            H 

*2                          bD    ^*     t»C    *-< 

d                   2  '&   0  •§ 

S              3  S  b  S 

S          l^w2 

la 

i 

i 

|  |i|  j|  J 

"  a  1          | 

r  §   fl                  rS 

03           C3                        o 

o*     £?    Q,          O   Sfi    'C             * 

^    "    °            .         ^ 

a 

cS 

•I    ^     a     *     I  •-  1 

§  s  °  »  1    '^ 

1 

1     2ll      5ljM 

^              0     S     g            OnT^^C^ 

v         -2T5T5         --H^aajS 

^                    _|00                                   cS^r^G 

3  llll      1 

"S1!:  4 

3 

.  g  1  a  1    *  *  f  ''  3 

^'sJa;^     -sI'lS10 

Iil«i  || 

ft 

CG 

8  &&S  §rfB^  S^ 

a  10  °  ^3  .«  .«    .c  a  a  o 

cJcsO    fto'CO'S    o3    cJi—  t 
02          CO                       55                Ofl 

200 


ELECTRICAL  EQUIPMENT 


fifl 

o^  a*>  0 
S  fl.2  2  S 

OO                  CO           i-H                                 lO 

<N                       ^             .rH                                          CO 

oJ-H  a® 

III                                              1 
CO                     1-H             O5    •>*                              O5 

i-H             O    CO                              lO 

£ 

S 

§ 

S 

J9 

1 

§                     ^            ^                                                              § 

o        8 

§§ 

o  «c  »o   -. 

<N   (N   IM    2 

« 
ci 

*"*                                                                                    CO 

§ 

0     -H 

^   ^   <N   0 

S 

1 

S     2 

^ 

0 

8 

oT 

V                   <o     00 

CD 

o 

e 

1 

a 

1         11 

a          a  a 

O 

5 

u 

V 
g          M 

S      M  8W 

<N 

08 

3 

3 

£      ,3 

^            3     ^ 

s 

a 

^o«       o 

1 

p 

"S 

>• 

Hi 

l!|i» 

K 

&i 

§ 

S  &w- 

CO           CO"  CO"  ^ 

(2  u 

||    ** 

O   33  w  ^  0} 

Full  scale  or 
normal  current 
or  voltage4 

«    &      iliS* 
10  a  ^        ^g1"0^^ 

^O                         >J.           >>IOC                -^ 

s^  mill  it 

Q   «   "                   ^-^<No£^               C^C 

2,200  to  4,000 
volts, 
10  to  100  amp. 
1,000  to 

5 

13 
> 

CO 

o 
"o 

a 

a 

03 

i-H 

1 

&. 

1|          |           If    2 

A              rd 

"I 

•gi 

1!  Sllll       i 

£          5 
1          1 

1  1 

c  a 

ft3    ^    2  -M    d                            W 

to                  M 

M           M 

M 

_       l£§j!j              1 

S         £ 

S     S 

o"--S 

S 

a 

1 

1 
1 

I 

g     s       s    § 
I    1    '  '  :  1  •  J               S 

i§dl-    1  .'J|             fgs 

iC    o  /K                o  -o  ~ 

^gQ             d«»oo                     |-|f;t« 
Q^!—,            ts-tJ^n110                      a   £    <-• 

r  i»  ?   !'i 

2     ^        i|  s-s             £     a 

J    1      |8|&               | 

For  series  arc  lamp  circuits. 
For  ground  detector  circuits, 

multiple-point  form,  1,  2,  3, 
and  4  point. 

For  ground  detector,  bayonet 
form. 
For  synchronizing. 

COSTS 


201 


Hill 

1 

*o  o  *o  o       ^o       *o       o 

fl 
o 

rH    0   <N    «0           CO           CO           t-' 

o 

1 

CJ     (B       I             CO 

"S                 7     S  *g          S 

5 

^  s       w  1  1    |  B 

• 

§*    C3    ^             Q)     *•* 

Jj 

•^    ®           o3     S 

>^    rQ                   '0      ,13                      §                   ,3         S 

^ 

| 

-t   Sf^-a.f-s 

1 

' 

tJgw>g£'5ofc£g 
^S^ajj-'S^g^jS 
PnrtpHtfH         H         H^ 

o 

.    ^.s 

I 

tlflll 

KATUS 

11  111 

o>  o  S 

•4 

<M 

PH 

CH 

«! 

||| 

N 

O 

fe 

°  ^'o 

"o         • 

0 

—  .'s  >• 

>   ^  a 

S 

g§8 

g  J  1 

S 

a 

CO           »O 

S 

03  § 

•g    -s    -g 

> 

"o  g 

"E    '1    '? 

X 

e  p, 

bjo        bo      r--^ 

3 

0 

^        3        ^ 
PH         PH         Q 

q 

M^  a 

H 

3^"" 

•-H^  ro 

«3     «  Ji 

§•2^"* 

a 

9 

1    S 
»                     §  ^ 

QO 

^^  i 

1 

<n            .            .  ^f  «« 

0) 

1         |         111 

13 

^       S       S  °  •*» 

| 

:• 

1        o       |1  S 

CHAPTER  XXII 
PROBLEMS 

PROBLEMS  ON  CHAPTER  I 

1.  Diagram  of  D.C.  Connections. — DATA:  A  shop  has  the  following 
220-volt  D.C.  motor  equipment: 

On  Feeder  No.  1. — Two  shunt  motors  driving  shafts  from  which  groups 
of  machines  are  driven. 

On  Feeder  No.  2. — Ten  shunt  motors,  each  driving  a  lathe,  and 
four  compound  motors  each  driving  a  planer. 

On  Feeder  No.  3. — Ten  compound  motors  for  individual  drive  for 
eight  punch  presses  and  two  shears. 


D.O.Oircuit 

(a) 


A.O. Circuit 


FIG.  107. — Representation  of  voltmeters  and  ammeters  on  D.C.  and  A.C. 

circuits. 

A  D.C.  switchboard  ammeter  is  usually  made  for  use  with  a  shunt;  an  A.C.  voltmeter  with 
a  voltage  transformer;  and  an  A.C.  ammeter  with  a  current  transformer.  Some  modifica- 
tions of  these  connections  are  discussed  in  the  text  and  used  in  later  problems. 

Lighting  Feeders. — The  shop  has  four  220-volt  lighting  feeders,  each 
ending  in  a  panel  board.  Each  panel  board  has  six  outgoing  circuits, 
and  each  circuit  is  controlled  by  a  two-pole  switch  and  protected  by 
a  two-pole  fuse-block. 

Generators. — Power  for  motors  and  lighting  is  furnished  by  two  220- 
volt' compound  wound  generators  which  are  to  be  run  either  singly  or 
in  parallel. 

REQUIRED. — Draw  a  diagram,  showing  directly,  or  indicating  by 
notes,  the  buses,  feeders,  and  other  connections  and  equipment  for  con- 
trolling the  motors  and  generators,  and  for  furnishing  power  to  the 
motors  and  lamps. 

202 


PROBLEMS  203 

Put  an  ammeter  in  series  with  each  generator,  and  each  feeder  circuit. 
Make  such  voltmeter  connections,  to  one  or  more  voltmeters,  that  the 
voltage  can  be  measured  across  the  buses  and  across  each  generator 
before  it  is  connected  to  the  buses.  (See  Fig.  107a,  for  method  of 
connecting  voltmeters  and  ammeters  in  the  diagrams.) 

2.  Diagram  of  A.C.  Connections. — DATA:  A  factory  has  the  following 
three-phase  440- volt  motors: 

On  Feeder  No.  1. — Ten  50-hp.  squirrel-cage  induction  motors,  driving 
shafts  for  group-driven  machines.  (See  Fig.  9,  page  31,  and  Fig.  23, 
c>  ft  page  49,  for  suitable  arrangements  of  auto-transformers  and  con- 
nections, to  obtain  low  voltage  for  starting  squirrel-cage  induction 
motors.) 

On  Feeder  No.  2. — Ten  100-hp.  synchronous  motors  driving  recipro- 
cating pumps.  The  motors  are  provided  with  squirrel-cage  induction 
motor  windings  (which  makes  them  self -starting),  in  addition  to  the 
regular  synchronous  motor  windings.  The  external  A.C.  connections 
are  the  same  as  for  squirrel-cage  motors.  In  addition  D.C.  connections 
are  necessary  to  excite  the  fields.  Power  is  obtained  from  the  exciter 
buses  for  exciting  the  fields  of  the  motors  as  well  as  of  the  generators. 

On  Feeder  No.  3. — Four  75-hp.  induction  motors  with  wound  rotors, 
for  conveyors. 

Lighting  Feeders. — Besides  these  power  feeders,  there  are  nine  110-volt 
60-amp.  single-phase  lighting  feeders,  obtaining  power  from  three 
lighting  transformers. 

Power  is  furnished  by  three-phase  440-volt  generators,  excited  from 
exciter  buses  by  two  exciters  which  are  connected  to  operate  either 
singly  or  in  parallel. 

For  solving  this  problem,  assume  the  following :  (a)  The  combined  kilo- 
watt capacity  of  all  the  generators  is  to  be  equal  to  the  total  kilowatt 
input  of  all  the  motors  at  rated  full-load,  assuming  a  motor  efficiency 
of  80  per  cent.;  plus  the  kilowatts  required  for  lighting. 

Power  factor  is  ordinarily  taken  into  account  in  determining  the 
size  of  A.C.  generators;  this  and  several  other  points  are  neglected  in 
the  present  problem,  but  are  considered  in  later  problems. 

(b)  At  least  two  generators  are  to  be  installed.     There  may  be  more 
than  two  if  the  total  power  required  is  so  great  that  each  generator 
has  a  capacity  of  1,000  kw. 

(c)  The  combined  capacity  of  all  transformers  is  to  be  equal  to  the 
rated  capacity  of  all  equipment  connected  to  their  secondaries. 

(d)  Three  transformers  are  to  be  installed,  unless  special  conditions 
make  some  multiple  of  three  desirable. 

All  these  assumptions  are  not  far  from  standard  practice.  They 
are  taken  up  more  fully  in  connection  with  later  problems. 

REQUIRED. — (a)  Determine  the  number  of  generators  and  the  size 
of  each,  and  the  size  of  each  transformer. 


204  ELECTRICAL  EQUIPMENT 

(6)  Draw  a  diagram  showing  all  connections,  fully  labelled.  Show 
only  one  ammeter  connection  and  one  voltmeter  connection  in  each 
circuit  in  which  you  consider  them  desirable  for  satisfactory  operation 
of  the  plant.  (In  practice  sometimes  there  is  provided  an  ammeter 
connection  in  each  line,  and  a  voltmeter  connection  across  each  pair 
of  lines.)  See  Fig.  1076,  for  meter  connections. 

PROBLEM  ON  CHAPTER  II 

3.  Safe  Equipment. — This  problem  is  to  be  based  not  only  on  the 
text  and  foregoing  problems,  but  also,  as  far  as  possible,  on  the  student's 
previous  observation. 

DATA. — A  small  shop  requires  power  for  motors  and  lighting.  This  is 
to  be  purchased  from  an  electric  power  company  whose  2,200-volt, 
three-phase,  three-wire  feeders  pass  the  building.  Induction  motors 
are  to  be  used  throughout  the  shop. 

REQUIRED. — What  voltage  would  you  specify  for  power?  For 
lighting?  What  provision  would  you  make  for  safety  to  equipment  and 
operators?  Draw  a  diagram  of  connections  from  the  2,200  volt  line 
to  one  or  two  motors  and  lamps.  Where  there  is  any  other  possible 
arrangement  of  equipment,  give  reasons  for  your  choice. 

PROBLEMS  ON  CHAPTER  III 

4.  Cement  Plant. — DATA:  A  cement  plant  uses  a  total  of  3,000  kw. 
for  power  and  lighting.     About  80  per  cent,  of  the  power  is  used  at  the 
main  plant,  which  adjoins  the  power  house.     The  remainder  of  the  power 
is  used  at  the  quarry,  which  is  1  mile  from  the  power  house. 

REQUIRED. — Answer  the  following  questions  and  give  reasons  for 
your  choice. 

(a)  Should  an  A.C.  or  a  D.C.  system  be  installed?  (If  you  consider 
both  A.  C.  and  D.C.  desirable,  for  what  purposes  should  each  be  used?) 

(6)  If  an  A.C.  system  is  selected,  what  should  be  the  number  of 
phases,  and  the  frequency? 

(c)  What  should  be  the  voltage  of  the  system?  (Or,  if  there  is 
more  than  one  voltage,  what  should  the  several  voltages  be?) 

6.  Machine  Shop. — DATA:  Power  for  a  small  machine  shop  is  to  be 
purchased  from  an  electric  power  company,  which  furnishes  it  at  2,200 
volts,  on  a  three-phase,  three-wire  system.  The  power  is  used  for 
lighting,  and  for  direct-connected  motors  driving  the  following  machines : 
6  engine  lathes,  2  planers,  2  drill  presses,  1  saw,  1  milling  machine,  1 
punch,  1  shear,  2  emery  wheels. 

REQUIRED. — Answer  the  same  questions  as  in  Problem  4.  (d)  If 
you  select  a  D.C.  system,  how  would  you  obtain  it?  (e)  If  you  select 
A.C.,  how  would  you  vary  the  speed  of  the  lathes  and  drill  presses? 

6.  Machine  Shop. — If  a  generator  is  to  be  installed  to  furnish  power 
to  the  shop  of  Problem  5,  how  would  the  questions  be  answered,  assum- 


PROBLEMS  205 

ing  that  the  generator  may  be  either  A.C.  or  D.C.,  and  of  any  desired 
voltage? 

7.  Machine  Shop. — DATA:  A  railroad  repair  shop  requires  power  for 
driving  a  large  number  of  engine  lathes,  boring  mills,  planers,  and  drill 
presses,  several  wheel  and  axle-lathes,  rivet  headers,  a  turn  table,  cranes, 
and  a  large  wood  shop.     The  wood  shop  is  about  2,000  ft.  from  the 
power  station.     The  turn  table  and  yards  are  to  be  lighted,  and  all  the 
buildings  are  to  be  lighted  inside. 

REQUIRED. — Answer  the  same  questions  as  in  Problems  4  and  5. 

PROBLEMS  ON  CHAPTER  IV 

8.  Pump. — DATA  :    A  motor  is  to  drive  a  reciprocating  pump,  pump- 
ing water  to  a  reservoir  at  a  height  of  200  ft.     The  pump  has  an  effi- 
ciency of  75  per  cent.,  and  is  required  to  deliver  100  cu.  ft.  per  min. 
continuously.    D.C.  power  is  available  for  the  motor.    (Weight  of  water 
is  62.5  Ib.  per  cu.  ft.) 

REQUIRED. — What  kind  and  size  of  motor  would  you  use? 

9.  Hoist. — DATA:    A  motor  is  used  to  drive  a  hoist  that  lifts  a  load 
of  10  tons  at  the  rate  of  100  ft.  per  min.     The  efficiency  of  the  hoist  is 
75  per  cent.     The  motor  is  used  for  1  hr.  at  a  time. 

REQUIRED. — What  kind  of  a  motor  would  you  use?  How  much  power 
must  it  deliver  for  1  hr.?  How  much  would  it  deliver  continuously? 

10.  Variable  Speed — Dust. — DATA:    A  motor  is  to  be  installed  in 
a  very  dusty  place.     It  must  be  large  enough  to  deliver  any  power  up 
to  25-hp.  continuously,  to  a  machine  that  runs  at  any  speed  from  25  to 
100  r.p.m.     A  220-volt  D.C.  system  is  available  for  power.     When 
the  machine  is  adjusted  for  a  certain  load,  it  should  maintain  a  speed 
that  is  approximately  constant.     Assume  that  the  efficiency  of  gear 
reduction  per  pair  of  gears  is  95  per  cent.,  taking  into  account  bearing 
friction.     The  gear  reduction  must  not  exceed  1:7  per  pair  of  gears.1 

REQUIRED. — (a)  Suggest  in  detail  a  suitable  motor-driving  mechan- 
ism (by  gear  or  other  mechanical  means) .  (6)  Give  as  full  information 
as  you  can,  by  which  the  motor  can  be  ordered,  (c)  What  current  will 
the  motor  take?  (d)  What  horsepower  can  it  deliver  when  the  en- 
closing lids  are  removed? 

11.  Variable  Torque  and  Speed. — DATA:    A  shunt  motor  drives  a 
shaft  at  various  speeds  as  required  and  is  to  exert  a  torque  at  each  speed 
as  follows : 

50  to  150  r.p.m.  4,000  Ib.-ft.  torque. 

151  to  300  r.p.m.  3,000  Ib.-ft.  torque. 

301  to  400  r.p.m.  2,000  Ib.-ft.  torque. 

1  See  paragraph  on  p.  166,  entitled  The  Best  Available  Speed  of  Motor. 
S.  15:  221;  23:  28-34. 
A.  pp.  612-615. 


206  ELECTRICAL  EQUIPMENT 

The  motor  must  be  large  enough  to  exert  this  torque  continuously. 
The  field  current  at  rated  speed  (without  field  rheostat)  is  2  per  cent, 
of  the  armature  full-load  current.  The  motor  is  connected  to  a  220- 
volt  B.C.  circuit. 

REQUIRED. — (a)  Suggest  a  satisfactory  arrangement  for  obtaining 
these  results;  (6)  Specify  the  motor  horsepower  and  the  maximum 
resistance  required  in  any  rheostats  that  are  to  be  provided. 

PROBLEMS  ON  CHAPTER  V 

12.  Small  Starting  Torque.— DATA:    A  machine  runs  at  200  r.p.m., 
and  requires  a  torque  of  800  Ib.-ft.     It  is  located  where  men  are  working 
who  are  not  familiar  with  electric  circuits.     A  2,200-volt,  three-phase, 
60-cycle  circuit  is  available  for  power.    No  speed  control  is  required,  and 
there  is  only  a  small  starting  torque.     The  speed  reduction  per  pair  of 
gears  must  not  exceed  1:6. 

REQUIRED. — Give  as  full  information  as  you  can  for  purchasing  the 
motor  that  you  would  install.  Give  reasons  for  your  choice. 

13.  Large  Starting  Torque. — DATA  :    A  220-volt,  three-phase,  60-cycle, 
25-hp.  squirrel-cage  induction  motor  runs  a  machine  having  large  start- 
ing friction;  so  that  the  motor  must  exert  a  starting  torque  that  is 
150  per  cent,  of  the  rated  running  torque. 

REQUIRED. — (a)  What  voltage  must  be  provided  for  starting?  If 
this  voltage  is  provided  by  an  auto-transformer,  what  is  the  starting 
current  in  the  motor,  and  in  the  line?  (6)  What  would  these  currents 
be  if  the  motor  started  at  27  per  cent,  of  full-load  torque?  Note  the 
advantage  of  starting  light  if  possible. 

14.  Large  Starting  Torque. — DATA:    The  same  as  in  Problem  13, 
except  that  a  phase-wound  motor  is  installed.     Assume,  in  accordance 
with  the  current  curve  in  Fig.  7,  that  above  full  load  the  primary 
current  is  nearly  proportional  to  the  torque. 

REQUIRED. — Find  the  starting  current  taken  from  the  line  at  150 
per  cent,  of  rated  running  torque,  and  compare  with  that  in  Problem  13a. 

15.  Pump. — DATA:    A  reciprocating  pump  is  to  be  operated  by  an 
A.C.  motor,  and  a  440- volt,  three-phase,  60-cycle  circuit  is  available. 
Four  plans  are  under  consideration  with  reference  to  the  pump  and 
motor  installation: 

(a)  According  to  the  first  plan  the  pump  is  to  be  connected  directly 
to  the  piping  system  pumping  into  the  reservoir,  so  that  in  starting 
it  must  overcome  its  own  starting  friction  and  the  inertia  of  the  column 
of  water,  and  in  addition  it  must  work  against  the  water  pressure. 

(6)  The  second  plan  is  that  the  pump  be  installed  in  or  near  the 
generating  station,  pumping  into  the  reservoir  as  before,  but  that  it 
be  provided  with  a  by-pass  for  the  water  making  it  possible  to  start 
with  but  little  torque  on  the  motor. 


PROBLEMS  207 

(c)  The  third  plan  is  the  same  as  the  second,  except  that  the  motor  is 
to  be  on  a  rather  long,  heavily  loaded  transmission  line  carrying  other 
loads  at  low  lagging  power  factor,  and  it  is  proposed  to  use  a  motor  that 
will  improve  the  power  factor  more  than  an  induction  motor  will  do. 

(d)  The  fourth  plan  is  the  same  as  the  first,  except  that  the  reservoir 
will  be  omitted,  so  that  fluctuations  in  the  amount  of  water  used  must 
vary  the  speed  of  the  motor. 

The  pressure  in  the  water  mains  is  60  Ib.  per  sq.  in.  The  average 
demand  for  water  is  1000  cu.  ft.  per  min.,  and  the  reservoir  is  large 
enough  to  take  care  of  all  fluctuations.  The  maximum  demand,  which 
will  last  for  an  hour  at  a  time,  is  4000  cu.  ft.  per  min.  The  pump 
efficiency  is  70  per  cent. 

REQUIRED. — Specify  the  kind  and  size  of  motor  to  be  provided  accord- 
ing to  each  plan. 

PROBLEMS  ON  CHAPTER  VI 

16.  Machine  Shop.— DATA  the  same  as  in  Problem  7. 
REQUIRED. — Work  out  a  different  system,  based  on  this  chapter, 

that  would  meet  all  requirements.     Is  this  preferable  to  the  arrange- 
ment worked  out  in  Problem  7?     Give  reasons. 

17.  Battery  Charging  from  A.C.    Power. — DATA:    A  factory  pur- 
chases electrical  energy  at  2  cts.  per  kw.-hr.     A  110- volt,  three-phase 
60-cycle  circuit  is  available  in  a  garage  of  the  factory,  and  is  to  be  used 
for  charging  automobile  batteries.     This  will  require  9.5  amp.  at  10 
volts,  for  each  of  fifteen  automobiles,  3  hr.  per  day,  6  days  in  the  week, 
throughout  the  year. 

REQUIRED. — Work  out  an  arrangement  for  charging  the  batteries, 
draw  a  diagram  of  connections,  and  estimate  the  cost  per  year  for 
electrical  energy. 

PROBLEMS  ON  CHAPTER  VII 

18.  A.C.  Transmission. — DATA  :    A  factory  has  a  440- volt  three-phase 
60-cycle  power  plant.     A  branch  of  the  factory  1  mile  distant  is  to 
use  100  kilowatts  at  90  per  cent,  power  factor.     Assume  that  the  line- 
drop  will  be  10  per  cent.,  line  reactance  being  negligible. 

REQUIRED. — What  would  you  specify  for  voltage  of  the  transmission 
Jine?  (See  Chapter  III.)  Specify  the  voltage  ratio,  size,  and  number 
of  transformers  to  be  installed  at  each  end  of  the  line,  and  draw  a 
diagram  showing  all  connections  from  the  buses  at  the  main  power  house 
to  the  substation  buses  at  the  branch  factory. 

19.  Machine    Shop. — DATA:     The    machines   in   a   machine   shop 
using  a  total  of  200  kw.  at  85  per  cent,  power  factor  are  to  be  driven  by 
A.C.  motors.     A  2,200-volt  two-phase  60-cycle  system  is  available  as  a 
source  of  power.     It  is  to  be  expected  that  this  will  later  be  changed 
to  a  three-phase  system. 


208  ELECTRICAL  EQUIPMENT 

REQUIRED. — What  system  would  you  adopt  for  the  motors?  Give 
full  information  for  purchasing  the  transformers. 

20.  Motor  Starter.— DATA:    One  of  the  machines  in  Problem  19 
requires  a  50-hp.  squirrel-cage  induction  motor,  which  starts  with  full- 
load  torque. 

REQUIRED. — Specify  the  number  of  auto-transformers  to  be  provided 
for  starting,  and  the  voltage  and  short-time  current  capacity  that  is 
required  in  each  winding  of  each  auto-transformer.  Draw  a  complete 
diagram  of  connections,  indicating  on  it  the  current  capacities  of  the 
several  parts  of  the  equipment. 

PROBLEMS  ON  CHAPTER  VIII 

21.  Storage  Battery  for  Off-peak  Load. — DATA:  An  office  building 
is  provided  with  a  power  plant,  in  which  the  generators  are  running 
during  the  day.     A  storage  battery  is  to  be  installed  to  furnish  power 
for  lighting  when  the  generators  are  shut  down.     The  battery  capacity 
must  be  sufficient  to  light  100  60-watt  lamps  for  6  hr.     An  end-cell 
switch  is  provided  for  regulation  of   the  voltage  on  discharge.     The 
lighting  of  the  building  is  from  a  110-  and  220- volt  three- wire  system. 
It  is  permissible  if  desired  to  arrange  switches  by  which  the  circuits 
are  changed  to  a  110- volt  two-wire  system  when  they  are  fed  from 
the  storage  battery. 

REQUIRED. — (a)  Specify  the  kind  of  battery  that  is  required,  the 
total  number  of  cells,  the  number  of  end-cells,  and  the  ampere-hour 
capacity  of  each  cell. 

(6)  Draw  a  diagram  showing  connections  to  the  battery,  and  all 
necessary  electrical  equipment  for  furnishing  power,  and  for  controlling 
and  protecting  the  battery. 

22.  Storage  Battery  Truck. — DATA:    A  battery  truck  is  to  be  used  for 
carrying  material  from  section  to  section  of  a  factory.     The  truck  must 
make  four  trips  daily,  over  a  distance  of  2  miles.     The  truck  must  be 
large  enough  to  carry  5  tons,  but  the  average  load  carried  will  not  exceed 
2  tons.     A  three-phase,  60-cycle,  220-volt  circuit  is  available  for  obtain- 
ing power  to  charge  the  storage  battery. 

REQUIRED. — (a)  Specify  the  kind  of  battery,  the  number  of  cells,  and 
the  ampere-hour  capacity  of  each  cell. 

(6)  Specify  the  kind  of  apparatus  that  you  would  use  for  charging, 
and  the  A.C.  and  D.C.  voltage  and  current  capacity. 

(c)  Draw  a  complete  diagram  of  connections  for  charging. 

PROBLEMS  ON  CHAPTER  IX 

23.  Lamp  Economy. — DATA:     Tables  I  and  II,  in  Supplement  to 
Bulletin  20  of  National  Lamp  Works. 

REQUIRED. — (a)  Compute  the  amount  of  light  in  lumens  per  dollar 
invested  in  lamps,  for  25-,  40-,  60-  and  100- watt,  110- volt  lamps,  and 


PROBLEMS  209 

plot  a  curve  with  sizes  of  lamps  (in  watts)  as  abscissse  and  lumens  per 
dollar  invested  in  lamps,  as  ordinates. 

(6)  Plot  a  similar  curve  on  the  same  sheet,  for  220-volt  lamps. 

(c)  Plot  on  the  same  sheet,  curves  between  watts  and  lumens  per 
watt  for  110-  and  220-volt  lamps.     (Make  these  curves  dotted,  or  other- 
wise distinguished  from  (a)  and  (6). 

(d)  Conclusions. — Of  these  eight  kinds  of  lamps,  which  is  the  most 
economical  to  use,  considering  only  the  cost  of  lamps?    Which  is  the 
most  economical,  considering  only  the  cost  of  energy?     (Obviously  other 
considerations  sometimes  make  it  economical  to  use  neither  of  these 
lamps.) 

24.  Shop  Lighting. — DATA  :  A  shop  50  ft.  wide  by  90  ft.  long  has  a 
lighting  system  consisting  of  four  rows  of  110- volt,  100- watt  Mazda 
lamps,  with  eight  lamps  in  a  row.  The  rows  are  uniformly  spaced,  and 
the  lamps  are  uniformly  spaced  in  each  row.  Dome  reflectors  are  pro- 
vided for  direct  lighting,  making  the  illumination  practically  uniform. 
Average  conditions  exist  as  to  the  effect  of  dust  on  the  reflectors. 

REQUIRED. — Find  the  illumination  on  the  working  plane, 

(a)  When  the  lamps  are  new  and  reflectors  clean. 

(6)  After  1,000  hr.  of  use  when  reflectors  are  clean. 

(c)  When  lamps  are  new,  but  reflectors  have  not  been  cleaned  for 
6  weeks,  and 

(d)  After  1,000  hr.  of  use,  when  reflectors  have  not  been  cleaned  for 
6  weeks. 


PROBLEMS  ON  CHAPTER  X 

25.  Size  of   D.C.   Conductor.— DATA:    A    D.C.   220-volt,    250-hp. 
motor  is  300  ft.  from  the  buses.     The  line  leading  to  the  motor  is  a 
rubber-covered  copper  cable.     Market  quotations  for  this  kind  of  wire 
are  on  the  20-ct.  base,  and  there  is  a  discount  of  50  per  cent,  on  the 
wire.     Net  cost  of  labor  and  supplies  is  assumed  to  be  the  same  for 
any  size  of  wire  that  will  be  used.     The  motor  is  running  24  hr.  per  day, 
365  days  per  year.     Energy  costs  1  ct.  per  kw.-hr. 

REQUIRED. — (a)  What  size  of  cable  is  required  for  safety? 

(b)  What  size  of  cable  must  be  used,  in  order  that  the  line  drop  at 
full-load  shall  not  exceed  10  per  cent.? 

(c)  What  is  the  most  economical  size  of  wire? 

(d)  Which  of  these  three  sizes  should  be  provided? 

26.  Size  of  D.C.  Conductor. — DATA  the   same   as  in  Problem  25, 
except  that  the  motor  is  in  operation  only  8  hr.  per  day,  6  days  in  the 
week,  throughout  the  year. 

REQUIRED. — Find   the   sizes  of   wire  for  safety,  voltage  drop,  and 
economy,  as  in  Problem  25. 

14 


210  ELECTRICAL  EQUIPMENT 

27.  Size  of  B.C.  Conductor. — DATA  the  same  as  in  Problem  26, 
except  that  the  motor  carries  full-load,  2  hr.  per  day;  one-half  load  3  hr.; 
and  one-quarter  of  full-load  3  hr.  per  day. 

REQUIRED. — Obtain  results  as  in  Problem  26. 


PROBLEMS  ON  CHAPTER  XI 

28.  A.C.  Line  Voltage   Regulation. — DATA:  A  single-phase  line  has 
a  resistance  of  1  ohm  and  reactance  of  1  ohm. 

REQUIRED. — What  is  the  decrease  in  voltage  caused  by  100  amp. 
flowing  in  the  line,  if  the  power  factor  of  the  load  is  100  per  cent.? 
80  per  cent.?  60  per  cent.?  (Use  the  approximate  solution.) 

29.  Size  of  A.C.  Conductor. — DATA  the  same  as  in  Problem  25,  except 
that  the  motor  is  a  three-phase,  60-cycle  induction  motor. 

REQUIRED. — What  size  of  wire  should  be  provided,  in  order  to  be 
large  enough  for  all  requirements?  [Suggestion:  Reactance  is  not 
inversely  proportional  to  area,  so  that  the  solution  for  size  of  wire  for 
allowable  line  drop  is  somewhat  complicated.  A  good  method  of  pro- 
cedure is  to  find  first  the  size  required  for  safety  and  economy,  and  then 
to  find  by  trial  whether  the  voltage  drop  is  excessive.  If  it  is  excessive, 
find  by  trial  a  larger  size  that  does  not  have  an  excessive  drop.  Use 
the  approximate  solution  in  computing  voltage  drop.] 

PROBLEMS  ON  CHAPTER  Xll 

30.  Generator  Compounding. — DATA:    A  550- volt  D.C.  generator 
furnishes  power  to  a  feeder  whose  center  of  distribution  is  2,000  ft. 
from  the  generator.     The  feeder  delivers  300  kw.     The  size  of  the 
feeder  is  500,000  cir.  mils. 

REQUIRED. — What  must  be  the  per  cent,  overcompounding  of  the 
generator,  to  maintain  constant  voltage  at  the  center  of  distribution? 

31.  Maximum  Demand. — DATA:    A  factory  has  in  it  20  200-hp. 
shunt  motors,  30  100-hp.,  50  25-hp.,   100  10-hp.,  and  100  5-hp.;  also 
400  60- watt  lamps.     The  demand  factor  during  the  daytime  is  55  per 
cent,  for  motor  loads,  and  40  per  cent,  for  lighting. 

REQUIRED. — How  much  power  is  demanded  of  the  generating  station? 

32.  Number  and  Size  of  Generators. — DATA  the  same  as  in  the 
numerical  example  on  page  98,  except  as  follows:     Interest  is  at  5^ 
per  cent.,  depreciation  5  per  cent.,  and  insurance  and  taxes  each   1 
per  cent.;  other  costs  for  switchboard,  wiring,  and  equipment  are  $6 
per  kw.  of  total  generator  capacity;  power  is  required  8  hr.  per  day, 
6  days  per  week. 

REQUIRED. — Find  the  best  number  and  size  of  generators,  and  the 
cost  per  kilowatt  for  power,  including  fixed  charges. 


PROBLEMS  211 

PROBLEMS  ON  CHAPTER  XIII 

33.  Size  of  Alternator  and  Engine. — DATA:    A  three-phase  engine 
type  generator  is  required  to  furnish  steady  power  to  200  1-hp.  in- 
duction motors,  running  at  full-load. 

REQUIRED. — The  kilovolt-ampere  capacity  of  the  generator,  and  the 
horsepower  of  the  engine  driving  it. 

34.  Combined    Load    at    Several    Power    Factors. — DATA:     The 
generator  of  Problem  33  is  also  required  to  furnish  power  to  four  100- 
hp.  induction  motors,  running  at  full-load,  and  fifty  100-watt  lamps. 

REQUIRED. — The  kilovolt-ampere  capacity  of  the  generator.  [Sug- 
gestion: Find  for  each  part  of  the  load,  the  power  component  of  the  kilo- 
volt-amperes  (=  kva.  X  power  factor),  and  the  "reactive"  component 
(=  kva.  X  sin  6,  if  cos  8  is  the  power  factor).1  Add  together  the 
power  components  to  find  the  total  power  component,  and  the  reactive 
components  to  find  the  total  reactive  component.  The  total  kilovolt- 
ampere  capacity  must  be  the  square  root  of  the  sum  of  the  squares  of 
the  two  components.] 

35.  Combined   Synchronous   and   Induction   Motor   Load. — DATA: 
The  motor  and  lighting  loads  are  the  same  as  in  Problem  34,  except 
that  some  of  the  induction  motors  are  replaced  by  synchronous  motors, 
whose  fields  are  adjusted  so  that  the  power  factor  of  the  generator  load 
is  100  per  cent. 

REQUIRED. — Find  the  necessary  kilovolt-ampere  capacity  of  the  gen- 
erator. 

PROBLEMS  ON  CHAPTER  XIV 

36.  Constant -current  Regulating  Transformer. — DATA:     A  constant- 
current  regulating  transformer  operating  from  a  2,200  volt  circuit  is 
to  furnish  power  for  100  60-watt  6.6  amp.  series  lamps.     The  line  drop 
in  the  circuit  is  25  volts.     The  power  factor  of  the  load  is  practically 
100  per  cent.     At  full-load,  the  transformer  efficiency  is  93  per  cent., 
and  the  power  factor  of  the  primary  84  per  cent. 

REQUIRED. — The  kilovolt-ampere  capacity,  and  the  primary  and 
secondary  current  and  voltage  capacities. 

37.  Feeder  Voltage  Regulator. — DATA:  A  three-phase  feeder  has  a 
voltage  of  2,300  at  the  buses,  a  line  resistance  of  0.1  ohm,  and  a  line 
reactance  of  0.25  ohm  per  line.     The  feeder  is  required  to  deliver  power 
from  zero  to  8  kw.,  at  any  power  factor  from  60  per  cent,  (lagging)  to 
100  per  cent.     An  induction  regulator  is  to  be  used  to  maintain  constant 
voltage  at  the  end  of  the  line. 

REQUIRED. — The  current  capacity,  the  boosting  voltage,  the  per 
cent,  boosting,  and  the  total  three-phase  kilovolt-ampere  capacity  of  the 
regulator. 

1  See  brief  table  of  sines  and  cosines  on  page  90. 


212  ELECTRICAL  EQUIPMENT 

PROBLEMS  ON  CHAPTER  XV 

38.  Instrument  Transformers. — DATA  as  in  Problem  37:    A  watt- 
hour  meter  is  used  to  record  the  energy  delivered  by  the  feeder.     The 
meter  has  100-volt  potential  windings  and  5-amp.  current  windings. 

REQUIRED. — What  should  be  the  theoretical  transformer  ratio  of  the 
current  and  voltage  transformers,  according  to  these  data? 

39.  Instrument    Transformers. — DATA:    If    voltage    and    current 
transformers  cannot  be  obtained  that  produce  exactly  100  volts  and 
5  amp.  respectively,  in  their  secondaries  at  rated  voltage  and  full- 
load,  the  next  higher  or  lower  rating  is  selected,  and  the  meter  is  cali- 
brated to  suit  (see  list  of  transformers  on  pp.  116,  120).    For  the  most 
accurate  meter  indications,  the  voltage  elements  of  the  meter  in  Problem 
38  should  have  between  90  and  125  per  cent,  of  the  rated  voltage,  and 
at  full-load  the  current  elements  should  have  between  75   and   150 
per  cent,  of  the  rated  current. 

REQUIRED. — Select  current  and  voltage  transformers  of  commercial 
sizes,  suitable  for  this  service. 

PROBLEMS  ON  CHAPTER  XVI 

40.  Switches. — DATA:  'A  220- volt  D.C.  generating  plant  contains 
four  1,000  kw. -generators,  and  the  following  power  feeders: 

Four  500  kw.  feeders. 
Four  300  kw.  feeders. 
Four  200  kw.  feeders. 

REQUIRED. — Give  as  full  information  as  possible,  as  to  the  switches 
to  be  installed  for  the  control  of  these  circuits. 

41.  Rheostats. — DATA:    A  25-hp.  motor  on  a  feeder  in  Problem  40 
is  to  run  normally  at  600  r.p.m.     Field  and  armature  rheostats  are  to  be 
provided  for  varying  the  full-load  speed.     There  are  to  be  enough 
steps  in  each  rheostat  to  vary  the  speed,  by  steps  of  100  r.p.m.,  from 
100  to  1,200  r.p.m. 

REQUIRED. — Give  all  possible  specifications  for  the  rheostats. 

42.  Balancer-set  Outfit. — DATA:    In  the  plant  of  Problem  40  are  a 
balancer  set  and  eight,  three-wire  15-amp.  lighting  feeders.     The  lighting 
system  is  so  arranged  that  the  unbalancing  of  the  current  will  not  exceed 
15  per  cent,  of  the  maximum  possible  lighting  current.     The  armature 
current  of  the  balancer  set  is  8  per  cent.,  and  the  field  current  2  per  cent, 
of  the  machine  full-load  current,  at  no  load. 

REQUIRED. — (a)  Give  full  information  as  to  the  switches  to  be  pro- 
vided in  addition  to  those  of  Problem  40. 

(6)  Specify  the  kilowatts  and  voltage  of  each  machine  of  the  bal- 
ancer set. 

(c)  Give  full  information  regarding  the  motor  starting  rheostat. 


PROBLEMS  213 

(d)  Draw  a  complete  diagram  of  connections  of  the  balancer  set. 
Note  that  if  the  neutral  line  is  closed  before  the  resistance  is  all  cut  out 
of  the  armature  rheostat,  the  voltage  is  unbalanced,  and  all  the  lamps 
on  one  side  of  the  circuit  may  be  destroyed.  Provide  if  possible  some 
device  making  this  condition  impossible. 

43.  Line-drop  Compensator. — DATA  as  in  Problem  37.    In  addition, 
a  voltmeter  and  a  voltage  regulating  relay  are  to  be  connected  to  current 
and  voltage  transformers  and  to  a  line-drop  compensator,  in  the  gener- 
ating station,  to  indicate  and  regulate  the  voltage  at  the  end  of  the 
feeder. 

REQUIRED. — (a)  Specify  the  per  cent,  compensation  of  the  com- 
pensator, and  all  transformer  ratios. 

(6)  Draw  a  complete  diagram  of  connections  of  instrument  trans- 
formers, compensator,  voltage  regulating  relay,  and  induction  voltage 
regulator.  [Suggestion:  The  diagram  may  be  simplified  by  notes 
referring  to  diagram  of  Problem  37.] 

PROBLEMS  ON  CHAPTER  XVII 

44.  Feeder  Protection  and  Voltage  Regulation. — DATA:   The  feeder 
of  Problem  37  is  to  be  protected  by  a  circuit -breaker  that  carries  the 
rated    full-load   without    excessive    heating.    The    circuit-breaker    is 
tripped  by  a  relay  that  operates  in  case  an  overload  occurs,  exceeding 
double  the  normal  current.     The  relay  receives  its  current  from  cur- 
rent transformers.     It  can  be  set  to  operate  on  any  current  from  3  to 
6  amp. 

Required :  (a)  Specify  the  current  and  voltage  rating  of  the  circuit- 
breaker,  and  the  current  transformer  ratio,  selecting  transformers  of 
customary  rating. 

(6)  Draw  a  diagram  of  connections  of  the  voltage  regulator,  circuit- 
breaker,  relay,  and  current  transformers  operating  the  relay. 

45.  Overload  Protection. — DATA  as  in  Problems  40  to  42. 
REQUIRED. — Specify  fully  the  equipment  for  overload  protection. 

PROBLEMS  ON  CHAPTER  XVIII 

46.  Lightning  Protection. — DATA  as  in  Problem  37.     In  addition,  the 
feeder  is  to  be  protected  against  lightning.    The  line  is  in  a  mountainous 
district  where  lightning  discharges  are  very  severe.     It  is  proposed  to 
put  arresters  at  each  end  of  the  line,  and  every  1,500  ft.  along  the  line. 

REQUIRED. — Select  suitable  lightning  arrester  equipment,  and  draw 
a  diagram  showing  the  complete  connections. 

47.  Lightning   Protection.— DATA  :     A  2,200-volt,   three-phase,   60- 
cycle  power  plant  furnishes  power  for  coal  mining.     Power  is  used 


214  ELECTRICAL  EQUIPMENT 

outside  each  mine,  for  machine  work,  pumping,  ventilating,  and  hauling, 
and  inside  the  mine  for  mining,  hoisting,  and  hauling.  Motor  generator 
sets  are  located  in  the  mines  and  elsewhere  as  required,  to  obtain  power 
at  275  volts,  D.C.,  for  hauling  and  other  purposes. 

REQUIRED. — Select  suitable  lightning  arrester  equipment,  and 
draw  a  diagram  showing  connections. 

PROBLEMS  ON  CHAPTER  XIX 

48.  Meters. — DATA  as  in  Problems  40  and  42. 

REQUIRED. — (a)  Draw  a  diagram  showing  the  circuits,  including  all 
meters  and  ground  detecting  and  meter  switching  apparatus,  and  specify 
the  full  scale  indication  of  each  meter. 

49.  Meters. — DATA  as  in  Problem  47.     There  are  three  generators 
in  the  power  plant. 

REQUIRED. — Lay  out  suitable  feeders,  and  show  all  necessary  meters, 
meter  switches,  ground  detecting  and  synchronizing  apparatus. 

PROBLEMS  ON  CHAPTER  XX 

50.  Machine  Shop  Motors. — DATA:  A  machine  shop  is  to  be  equipped 
with  motors  for  individual  drive  for  the  following  machines: 

Engine  lathe,  16-in.  swing,  for  heavy  duty. 
Engine  lathe,  12-in.  swing,  for  average  duty. 
Planer,  6-ft.  bed,  42  in.  wide  between  housings. 
Boring  mill,  6  ft.  diameter  of  table,  for  average  duty. 
Pair  of  emery  wheels,  16  in.  diameter,  for  heavy  duty. 
Vertical  drill  press  (upright  drilling  machine),  32-in.  table. 
Punch  press  for  punching  a  1,^-in.  hole  in  J^-in.  soft  steel. 
Lever  shear  for  cutting  y±  in.  by  48-in.  stock. 

REQUIRED. — Specify  the  horsepower  of  motor  for  each  application. 

51.  Crane — Hoisting  Motor. — DATA:    A  50-ton  travelling  crane  is 
required  to  hoist  full-load  at  15  ft.  per  min.,  and  to  lower  it  at  the  same 
speed,  with  dynamic  braking.     The  motor  is  hoisting  one-third  of  the 
time,  lowering  one-third,  and  idle  one-third. 

REQUIRED. — (a)  What  power  is  the  motor  required  to  deliver  during 
hoisting? 

(6)  What  is  the  equivalent  power  during  lowering,  in  its  effect  in 
heating  the  motor? 

(c)  What  size  of  motor  is  required? 

52.  Crane — Trolley  Motor. — DATA  as  in  Problem  51.     In  addition, 
the  weight  of  the  trolley  is  20  tons;  maximum  trolley  speed  is  120  ft.  per 
min.;  average  acceleration  and  retardation  is  4  ft.  per  sec.  per  sec. 
The  trolley  is  accelerating  during  one-third  of  the  time,  retarding  during 


PROBLEMS  215 

one-third,  travelling  uniformly  during  one-sixth,  and  either  stationary 
of  drifting1  (without  power)  during  one-sixth. 

REQUIRED. — (a)  The  power  required  during  steady  travel. 

(6)  The  power  required  during  acceleration. 

(c)  The  equivalent  of  the  power  during  lowering,  in  its  effect  in 
heating  the  motor. 

(d)  The  size  of  trolley  motor. 

53.  Crane — Bridge  Motor. — DATA  as  in  Problems  51  and  52.     In 
addition,  the  weight  of  the  bridge  is  40  tons;  maximum  bridge  velocity 
is  300  ft.  per  min.     No  power  is  used  during  retardation,  and  accelera- 
tion is  small  enough  so  that  power  during  acceleration  is  practically  the 
same  as  during  steady  travel.     Period  of  acceleration  and  steady  travel, 
30  sec.     Period  of  retardation  and  rest,  45  sec. 

REQUIRED. — (a)  The  power  required  during  steady  travel. 

(b)  The  size  of  the  motor.  [Suggestion:  No  variation  of  power  is 
considered,  except  as  it  changes  from  zero  to  the  maximum.  Since 
there  is  only  one  value,  the  expression  for  intermittent,  instead  of  variable 
power  can  be  used,  if  desired.] 

54.  Acceleration  of  Motor  Rotation. — DATA  as  in  Problem  52.     In 
addition,  the  weight   of   the   gear  and  motor  armature  is  1,000  lb.; 
outside  diameter  of  the  armature  is  16  in.;  diameter  of  trolley  track 
wheels  is  10  in.;  and  gear  ratio  from  motor  to  track  wheel  is  2: 1. 

REQUIRED. — How  much  additional  power  is  required  during  accelera- 
tion, on  account  of  accelerating  the  rotation  of  the  armature  and 
gear? 

GENERAL  PROBLEMS 

55.  Railroad  Repair  Shop. — DATA:    A  small  railroad  repair  shop 
(see  Fig.  108)  is  to  be  provided  with  D.C.  power  for  motors,  lighting 
and  battery  charging,  for  the  following  equipment: 

Motor  drive  for 

Four  26-in.  lathes  for  heavy  duty. 

Three  504n.  lathes  for  average  duty. 

One  lathe  used  on  machinery  steel — cutting  speed,  60  ft.  per  min.,  %-in. 
cut;  ^-in.  feed. 

One  lathe  used  on  soft  cast  iron — cutting  speed,  40  ft.  per  min.;  ^-in. 
cut;  Y^-m.  feed. 

One  wheel  lathe  for  heavy  duty  on  84-in.  wheels,  with  separate  motor 
for  tail  stock. 

One  12-ft.  boring  mill. 

One  6-ft.  boring  mill. 

1  Strictly,  if  it  is  drifting  without  power,  it  is  losing  velocity,  so  that 
the  power  during  retardation  will  be  less.  Practically  the  error  is  small  in 
assuming  that  there  is  no  change  of  velocity  during  a  small  amount  of 
drifting. 


216 


ELECTRICAL  EQUIPMENT 


One  36  in.  X  8-ft.  planer  for  average  duty. 
One  42  in.  X  16-ft.  planer  for  average  duty. 
One  60  in.  X  20-ft.  planer  for  average  duty. 
One  14-in.  slotter. 
One  %  in.  X  96-in.  shear. 


2    1 


§ 
I 


One  multiple-spindle  drilling  machine,  for  10  1-in.  drills. 
One  radial  drill  for  heavy  duty,  10-ft.  arm. 
Two  upright  drills,  22-in.  table. 
One  16-in.  shaper. 
One  2>£-in.  bolt  cutter. 


PROBLEMS  217 

One  2-in.  stucTcutter  (same  power  required  as  for  the  same  size  of  bolt 
cutter). 

One  6-in.  pipe  cutter. 

One  16-in.  emery  wheel. 

One  60-ton  hydrostatic  wheel  press. 

One  small  hydrostatic  press  requiring  5-hp.  motor. 

One  80-ton  crane — hoisting  and  lowering  5  ft.  per  min.;  trolley  travel  30 
ft.  per  min.;  bridge  travel  100  ft.  per  min.;  acceleration  negligible;  weight 
of  trolley,  25  tons;  weight  of  bridge,  70  tons.  Each  motor  is  working  only 
a  few  minutes  out  of  an  hour. 

One  100-ft.  turn  table  requiring  35-hp.  motor. 

Lighting. — General  illumination — also  special  illumination  as  required, 
in  pits,  inside  locomotives,  etc. 

(a)  In  the  main  shop. 

(b)  In  the  round  house. 

(c)  On  the  turn  table. 

(d)  In  the  yards. 

Battery  charging  for  train  lighting,  for  two  trains  each  composed  of 

One  baggage  car. 

One  60-ft.  mail  car. 

Two  16-section  Pullman  sleepers. 

Four  coaches. 

In  addition  to  other  demands,  the  batteries  on  the  Pullman  cars  are 
to  be  adequate  for  lighting  2  hr.  before  the  train  is  made  up,  and  1  hr. 
at  the  end  of  the  trip. 

The  total  time  of  the  trip  is  8  hr.  in  each  direction.  Each  train  makes 
the  round  trip  every  day.  If  the  axle-generator  system  is  adopted,  it 
is  still  necessary,  on  account  of  frequent  and  long  stops,  to  give  the 
batteries  a  charge  at  the  end  of  each  round  trip,  sufficient  for  2  hr. 
service  after  the  train  starts. 

Generators,  Switchboard  Equipment  and  Wiring. — For  corresponding 
sizes  of  generators,  the  cost  of  generators  and  other  equipment  per  cent, 
fixed  charges,  and  cost  of  energy  are  as  in  Problem  32.  The  demand 
factor  of  the  total  load  is  55  per  cent.  The  plant  is  in  operation  24 
hr.  per  day,  300  days  per  year. 

REQUIRED. — Draw  a  complete  diagram  of  connections  (condensed  by 
notes);  select  the  voltage  or  voltages  of  the  system;  specify  the  kind  of 
motor  for  each  application  and  horsepower  of  each;  the  number,  size 
and  kind  of  generators;  the  size,  kind  and  voltage  of  each  machine  of 
each  motor-generator  set;  and  the  size  and  location  of  all  lamps.  Give 
all  possible  information  regarding  meters,  switches,  circuit-breakers, 
rheostats,  and  other  electrical  equipment  that  should  be  provided  in 
the  power  plant  and  shop,  and  the  size  and  number  of  cells  in  each 
battery. 

66.  Cement  Making  Plant. — DATA:  The  mechanical  process  of 
cement  making  is,  in  brief,  as  follows:  Limestone  and  shale  are 


218  ELECTRICAL  EQUIPMENT 

quarried,  crushed,  dried  and  stored  ready  for  use.  They  are  then  mixed 
in  suitable  proportions,  and  reduced  by  several  additional  processes 
to  a  fine  powder.  This  powder  is  passed  through  a  kiln  and  melted  to 
a  clinker  formation,  and  certain  chemical  changes  take  place.  The 
clinker  is  then  sometimes  exposed  to  the  weather  for  a  few  weeks,  but 
this  weathering  may  be  eliminated.  A  small  amount  of  gypsum  is 
then  added,  and  the  material  is  again  pulverized.  It  is  then  in  its  final 
state  and  is  stored  ready  for  shipment. 

Following  is  a  list  of  machines  and  operations  requiring  motors  in 
a  typical  cement-making  plant;1  the  list  is  in  the  order  of  handling  the 
cement  (see  Fig.  109).  Wherever  5  hp.  or  less  is  required,  a  5-hp. 
motor  is  installed  for  simplicity  of  layout,  and  to  reduce  the  number  of 
spare  motors  to  be  kept  on  hand.  Where  the  horsepower  is  not  given  in 
the  list,  it  is  to  be  worked  out  as  a  part  of  the  problem. 

In  the  quarry,  1,  3a,  85,  crushers  (150,  75  and  30  hp.;  2,  4,  belt 
elevators  (capacity  100  tons  per  hour,  lift  60  and  40  ft.  respectively); 
and  5,  belt  conveyor  (5  hp.). 

In  the  dryer  department,  6,  tram  conveyor  (10  hp.);  7,  rotating  the 
cylindrical  dryer  (25  hp.);  8,  belt  elevator,  (10  hp.);  and  9,  belt  conveyor 
(5  hp.)  to  separate  storage  tanks  for  shale  and  for  limestone. 

In  the  mix  department,  10a  and  106,  belt  conveyors  for  shale  and  lime- 
stone, respectively  (5  hp.  each)  to  the  mixing  bin;  11,  belt  elevator 
(5  hp.)  to  the  ball  mills. 

In  the  raw  department,  120,  12b,  ball  mills  (75  hp.  each);  13,  belt 
conveyor  and  belt  elevator  (5  hp.);  14a,  146,  14C,  Kent-Maxecon  mills 
(50  hp.  each);  15,  screw  conveyors  (5hp.)  160,&,  belt  elevators  (5  hp.); 
170,h,  tube  mills  for  raw  material;2 18,  belt  elevator  (5  hp.);  19,  20,  belt 
conveyors  (10  and  5  hp.  respectively)  to  the  kilns. 

In  the  kiln  department,  21  a  to  21e,  rotating  the  kilns  (20  hp.  each); 
22,  Peck  carrier  (10  hp.);  23,  belt  conveyor  (5  hp.)  to  clinker  storage 
department. 

In  clinker  storage  department,  24,  25,  belt  conveyors  (5  hp.  each); 
26,  belt  elevator  (5  hp.);  27,  belt  conveyor  (5  hp.);  28,  rolls  (10  hp.); 
29,  cable  (75  hp.). 

In  the  clinker  department  the  processes  are  the  same  as  in  the  raw 
department,  beginning  with  the  Kent-Maxecon  mills — Nos.  14  to  20. 

In  the  coal  department  are  one  60-hp.  and  one  25-hp.,  two  10-hp.  and 
three  5-hp.  squirrel-cage  motors. 

In  the  machine  shop  are  located  one  15-hp.  motor  for  line  shaft  and 
one  25-hp.  motor  for  air  compressors. 

In  the  stock  house  are  five  15-hp.  and  two  5-hp.  squirrel-cage  motors. 

1  In  nearly  all  details  the  motors  listed  are  as  in  the  plant  of  The  Cayuga 
Cement  Corporation,  Portland  Point,  N.  Y.;  information  was  furnished 
by  courtesy  of  Mr.  W.  H.  Kniskern,  General  Manager.     A  few  small  motors 
have  been  omitted  from  the  list,  but  none  that  would  affect  the  layout  of 
the  system. 

2  Assume  data  as  on  p.  182,  unless  otherwise  specified. 


PROBLEMS 


219 


- 


Mr 


-  v       .:..    -- 


•  «      •  7 


: 


A.C- 


222 


INDEX 


Compressors:  ammonia,  motors  for, 

168,  177 

air,  motors  for,  167,  177 
Condenser  lightning  arrester,  144 
Conductors,  see  Wires. 
Connection  diagrams,  see  Diagrams 

of  Connections. 
Constant-current    lighting     circuit, 

164 

Constant-current  regulating    trans- 
formers, 109 

Contact-making  voltmeter,  127 
Continuity  of  service,  9 
Controlling    and    regulating    equip- 
ment, 121-129 
Control  switches  or  controllers:  123 

applications,  124 
Conventional      representation       of 

equipment,  5 

Converters,    see    Synchronous   Con- 
verters. 

Conveyors,  power  for,  176 
Copper  wires,  data  on,  80-84 
Costs:  185-201 

choke  coils,  199 

circuit-breakers,  196-198 

copper  wires,  81,  84 

energy,  affecting  size  of  wires,  76 

generators,  185 

graphic  meters,  190 

integrating  meters,  189 

lightning  arresters,  199 

meters,  186-191 

motors,  185 

plug  and  instrument  switches, 

200 
relays:  bell-ringing,  194 

protective,  193 
steam  engines,  185 
switches,  195 
transformers,   constant  current 

regulating,  200 
instrument,  191 
power  and  lighting,   198 
turbo-generators,  185 
voltage  regulators,    200 
wiring,  84 

Cranes,  motors  for:  167,  174,  178 
Problems  51-55;  214,  217 


Current  carrying  capacities  of  con- 
ductors for  safety,  79.  80 

Current  transformers,  see  Trans- 
formers, Instrument. 

Curve-drawing  meters,  164 


D.C.  versus  A.C.  systems,  15 
Delta  connection  of  transformers,  49 
Demand  factor:  96 

Problem  31;  210 
Demand  meters,  164 
Detectors,  ground,  151 
Diagrams  of  connections:   balancer 

sets,  D.C.,  39 

conventional  representations,  5 
dynamotors,  42 
end-cell  switches,  55 
exciters,  2,  127 
generators:  A.C.,  2,  5,  101,  108, 

110,  112 
D.C.:  2,  5 

three-wire,  40 

instrument  switches,  3,  159-164 
lightning  arresters,  142-146 
meters,  148,  149,  151,  159-165, 

202 

motor-generators,  35,  39,  41 
motors:  A.C.,  5,  31,  35 

D.C.,  5,  41 

Problems  1,  2;  202,  203 
rectifiers,  37 

relays,  127,  129,  139-141 
rheostats  and  D.C.  motor  start- 
ers, 2,  5,  35,  36,  39-42,  108, 
127 

rules  for  making,  3-6 
starters,  induction  motor,  31 
switches    and    circuit-breakers, 
2,     5,     122-124,     134-136, 
139-141 

synchronous  converters,  36 
transformers     and     auto-trans- 
formers, 47,  49,   110,   115, 
116,  118,  119,  124 
typical  A.C.  and  D.C.  circuits,  2 
voltage  regulators,  112 
Ward  Leonard  system,  41 


INDEX 


223 


Diameters  of  wires,  80,  82 
Direct  lighting,  68 
Disconnecting  switches:  123 
applications,  124 
costs,  195 

Distribution  of  candlepower,  63,  64 
Distribution  systems:  A.C.,  85-91 

circuits  of,  1 

D.C.,  71-84 

requisites  of,  7 
Duplicate  equipment,  9 
Dust,  effect  of,  on  illumination,  66 
Dynamic  braking,  179 
Dynamos,  see  Generators. 
Dynamotors,  42 

E 

Economical  size  of  conductor,  74-78, 

91 

Efficiency:  auto-transformers,  47 
generators,  A.C.,  103 

D.C.,  93 

motor-generators,  38 
motors:  D.C.,  24 

induction,  30 
rectifiers,  38 

synchronous  converters,  38 
transformers,  45,  46 
utilization  of  illumination,  65 
Electrical  conductors,  see  Wires. 
Electrolytic  lightning  arrester,  145 
Enclosed  motors,  see  Motors,  D.C., 

Enclosed. 

End-cell  switch,  55,  56 
Engines,  steam,  costs,  185 
Exciters:  rheostats  for,  108 

and  exciter  circuits,  representa- 
tion of,  2,  4 

steam-  and  motor-driven,  107 
Expansion,  allowance  for,  13 


Feeders:  parallel,  protection  of,  140 

representation  of,  2,  4 

voltage  regulators  for,  111 
Field:  discharge  resistance,  107 

rheostats,  26,  124-126 

switches:  107 
plug,  cost,  200 


Fixed  charges,  10,  11 
Foot-candle,  illumination,  61 
Frequency:  choice  of,  17 

meters,   151 

variation  affecting  meter  accur- 
acy, 154 
Fuses,  130 


Gas  engines,  ignition,  58 
Generators,  A.C.:  100-108 
characteristics,  103 
classifications,  100-103 
connections,  2,  106 
efficiency,  103 
equipment  of  circuits,  106 
excitation,  107 
frequency,  101 

number    and    size    required, 
Problem  2;  203.     See  also 
Generators,  D.C. 
phases,  100 

Problems  2,  33-35;  203,  211 
rating,  104 
regulation:  103,  104 

of  prime  movers,   104 
requisites  for  plant   operation, 

104 

revolving  field  or  armature,  102 
rheostats,  108 

speed  and  prime  mover,  102 
synchronizing,  105 
voltage:  102 

and   power-factor   adjust- 
ment, 108 
Generators,    A.C.  and  D.C.:   costs, 

185 

representation  of,  5 
voltage    regulators   for,    107, 

126 
Generators,  D.C.:  92-99 

adjustment  of  compounding, 

94 

available  sizes,  96 
characteristics,  92-94 
circuits  and  equipment,  2,  95, 

96 

connections,  95 
efficiency,  93 


224 


INDEX 


Generators  D.C.:  number  and  size 

required,  96-99 
parallel  operation,  94 
Problems  30-32;  210 
rating,  94 
regulation,  94 
temperature  rise,  94 
three-wire,  40 
Generator  circuits,  representation  of, 

2,  4 

Glare,  67 
Graphic  meters:  164 

costs,  190 

Ground  detecting  lamps,  152 
Ground  detectors:  151 

switches,  161 
Ground  wire  for  lightning  arresters, 

147 
Group  drive,  166 


H 


Hoists,  motors  for:  167,  175,  178 

Problem  9;  205 
Horn-gap  lightning  arrester,  143 


Ignition,  gas  engine,  58 
Ilgner  system  for  motor  speed  ad- 
justment, 40 
Illumination:  61-70 

computations,  68 

direct  and  indirect,  68 

intensity,  61,  66 

of  power  plant,  14 

Problems  23,  24,  55;  209,  215 

special,  69 

train  lighting,  58,  59 

utilization  efficiency,  65 
Impedance  drop,  see  Line  Drop. 
Indicators,  see  Meters,  Lamps. 
Indirect  lighting,  68 
Induction  voltage  regulator,  111 
Industrial   applications  of   motors: 
167,  170 

Problems  50-56;  214-220 
Instrument  switches,  see  Switches. 
Insrument  transformers,  see  Trans- 
formers, Instrument. 


Insulated      wire,      diameters      and 

weights,  80,  82 
Integrating  meters:  164 

costs,  189 

Intensity  of  illumination,  61,  66 
Intermittent  loading  of  motors,  183 

K 

Kelvin's  Law,  75 

Knife  switches,  see  Switches. 


Labels  on  diagrams,  6 
Lamps:  for  synchronizing,  150 

ground  detecting,  152 
Layout  of  power  station,  14 
Light  flux,  62 
Lighting:  61-70 
automobile,  58 
circuits:  D.C.,  line  drop,  71 
frequency  for,  18 
voltage  for,  19 
of  power  plant,  14 
Lightning  arresters:  142-147 
choke  coils  for,  145 
costs,  199 

ground  wire  and  ground  con- 
nections, 147 
Problems  46,  47;  213 
relative  merits,  145 
Line  drop :  see  also  Resistance  of  Con- 
ductors   and   Reactance    of 
Transmission  Lines. 
A.C.:  85-90 

affected  by  power  factor,  86 
single-phase,  85 
three-phase,  87 
two-phase,  90 
vector  diagrams,  85,  86,  89 
compensator:  128 
Problem  43;  213 
D.C.:  71-74 

ground  or  rail  return,  73 
multiple  voltage,  74 
two-wire,  72 
reactance,  81,  83 
voltage  regulation,  90 
Locomotives,  battery,  58 
Lumen,  62 


INDEX 


225 


M 


Machine  tools,  motors  for:  167,  170, 

178 

Problems  50,  55;  214,  215 
Magnetic  blowout  lightning  arrester, 

144 

Maximum  demand  meters,  164 
Measuring  and  indicating  apparatus, 
see  Meters,  Lamps,  Switches. 
Mechanical  rectifiers,  see  Rectifiers. 
Mercury  rectifiers,  see  Rectifiers. 
Mertz-Price  system  of  relays,  141 
Meters:  148-165 

accuracy  with  instrument  trans- 
formers, 155 
applications,  162-165 
characteristics,  152 
constant-current    lighting    cir- 
cuit, 164 
costs,  186-191 

effect  of :  low  power  factor,  155 
stray  field,  155 
varying  frequency,  154 
varying  voltage,  155 
frequency,  151 

graphic  or  curve-drawing,  164 
ground  detectors,  151 
integrating,  164 
maximum  demand,  164 
power  factor,  149 
Problems  48,  49,  55,  56;   214- 

220 

scales,  152 
synchronism  indicators  or  syn- 

chronoscopes,  150 
watt-hour,  149,  164 
watt  meters,  148 
switches,  see  Switches. 
Motor   circuits:  frequency  for,   17, 

18 

line  drop,  71 
voltage  for,  20 
Motor-generators :  34-42 
efficiency,  38 
Problems  16,  17;  207 
induction  vs.  synchronous  mo- 
tors, 35 

vs.  synchronous  converters,  36 
15 


Motors,  A.C.:  28-33 

suitable  applications,  33 

types  available,  28 

induction :  adapted  to  location, 

29 
connections  for  starting,  31, 

49 

data  at  starting:  31,  32 
Problems  12-15;  206 
efficiency,  30 

operation  at  various  loads,  29 
power  factor,  30 
slip,  30 

speed  adjustment,  33 
speed  regulation,  32 
speeds,  usual,  30 
voltage,  frequency  and  phases, 

28 

Motors,  A.C.  and  D.C.: 
applications:  166-184 

Problems   3-16,  50-56;  204- 

207,  214-220 
available  sizes,  166 
costs,  185 

for  group  drive,  166 
kinds  for  various  applications, 

167 

representation  of,  5 
speed  affecting  rating,  183 
variable  and  intermittent  load- 
ing, 183 
sizes   for  various   applications, 

170-184 

Motors,  D.C.:  22-27 
efficiency,  24 

intermittent  and  variable  load- 
ing, 23 

loading  at  high  speeds,  23 
enclosed :  change  of  rating,  183 
Problem  10;  205 
required  in  some  cases,  22 
overloading,  23 
rating,  23 

speed  adjustment:  25 
by  multiple  voltage,  40 
by  Ward  Leonard  and  Ilgner 

systems,  40 
Problems  10,  11;  205 
regulation,  24 


226 


INDEX 


Motors,  D.C.:  speeds,  usual,  24 

voltage,  22 

Multigap  lightning  arrester,  142 
Multipath  lightning  arrester,  144 
Multiple  voltage  for   D.C.   motor- 
speed  adjustment,  26,  40 

N 

National    Electrical     Code:    (foot- 
note), 9 

sizes  of  conductors,  79,  80 
Notes  and  labels  on  diagrams,  6 
Number  of  -phases,  16 


Oil  circuit-breakers,  costs,  196-198 
Oil  switches,   see   Circuit  Breakers, 

Oil,  Non-automatic. 
Operating  cost,  10,  11 
Overload  relays,  see  Relays,  Protective. 


Parallel  feeders,  protection  of,  140 
Phases,  number  of,  16 
Plug  switches,  see  Switches. 
Point-by-point  computation,  68 
Potential    regulators,    see     Voltage 

Regulators. 

Potential   transformers,  see    Trans- 
formers, Instrument. 
Power  factor:  affecting  line  drop,  86 
affecting  meter  accuracy,  155 
brief  table  of  sines  and  cosines,  90 
meters,  149 
Power  plants:  circuits  of,  1 

requisites  of,  7 

Protective  equipment,  130-147 
Protective  relays,  see  Relays,  Pro- 
tective. 

Pumps,  motors  for:  168,  177 
Problems  8,  15;  205,  206 

R 

Reactance  drop,  see  Line  Drop. 
Reactance  of  transmission  lines,  81, 83 
Receptacles,     voltmeter     and     am- 
meter, see  Switches. 
Rectifiers:  34-38 
connections,  37 
efficiency,  38 


Rectifiers:  Problems  16,  17;  207 
suitable  applications,  37,  38 
Reflectors:  distribution  curves,  63-65 
effect  of  dust,  66 
effect  of  reflectors,  62 
types,  62,  63 
Refrigerating    plants,    motors    for, 

168,  177 
Regenerative    control,    or    dynamic 

braking,  179 

Regulating    transformers,    109-113, 
see  also  Voltage  Regulators. 
Regulation:  line,  see  Line  Drop  and 
Voltage  Regulators,  also  Con- 
stant-current   Circuits    and 
Transformers. 
generator:  A.C.,  103,  104 

D.C.,  92 
motor:  A.C.,  32 

D.C.,  24 

Regulators,  voltage,  seeVoltage  Regu- 
lator. 
Relay,  voltage  regulating,  127 

switches,  128 
Relays : 

auxiliary:  relay  switch,  128 
bell-ringing,  costs,  194 
time-limit,  costs,  194 
protective:  136-141 
applications,  139 
costs,  193 
overload,  137 
time-limit,  137 
Z-connection,  140 
voltage  regulating,  127 
Representation  of  equipment,  5 
Resistance:  drop,  see  Line  Drop. 
of  aluminum  conductors,  81,  83 
of  copper  conductors,  81,  83 
Rheostats:  124-126 

applications  and  specifications, 

124-126 

alternator  and  exciter  field,  108 
D.C.  generator  field,  96 
D.C.  motor,  for  speed  control,  25 
field,  representation  of,  5 
Problems    11,    41,   42,   55,   56; 

206,  212,  217-220 
Rolling  mills,  motors  for,  168,  177 


INDEX 


227 


Rotary  converters,  rotary  trans- 
formers, see  Synchronous 
Converters. 

Rubber-covered  wires,  current  carry- 
ing capacities,  80 


Safe    carrying    capacities    of    con- 
ductors, 79,  80,  91 
Safety  to  operators  and  equipment:  8 

Problem  3;  204 
Scott  connection  of  transformers  and 

auto-transformers,  48-50 
Screw  conveyors,  power  for,  176 
Segment  of  circle,  center  of  gravity, 

182 

Semi-indirect  lighting,  68 
Series  lighting  circuit,  110 
Series  transformers,  see  Transform- 
ers, Instrument. 
Service,  continuity  of,  9 
Shadows,  affecting  illumination,  67 
Shunt     transformers,     see     Trans- 
formers, Instrument. 
Size  of  conductor:  based  on  allow- 
able line  drop,  71,  85 
based  on  economy,  74 
for  safety,  79,  80,  82 
with  variable  current,  78 
Solid  conductors,  see  Wires. 
Spacing  of  lamps,  67 
Speed :  adjustment  of  D.C.  motors,  25 
variation   of   motors,    affecting 

rating,  183 
Station  layout,  14 
Steam  engines,  costs,  185 
Steel  rolling  mills,  motors  for,  168, 

177 
Storage  batteries :  51-60 

capacity  on  heavy  discharge, 

53 

charge    and    discharge    vol- 
tages, 54 
comparison  of  various  kinds, 

51-55 

construction,  51 
cost,  51 

current  charging  rate,  53 
current  discharging  rate,  52 


Storage    batteries:    durability    and 

repairs,  52 
efficiency,  54 
end-cell  switch,  55,  56 
Problems  17,  21,  22,  55;  207, 

208,  217 

space  occupied  and  weight,  52 
types,  51 

Storage  battery  applications:  auto- 
mobiles, trucks,  battery 
locomotives,  58 
battery  substation,  56 
generating  station,  55 
on  separate  circuits,  56,  57 
portable  service,  57-60 
stationary  service,  55-57 
train  lighting,  58-60 
Stranded  conductors,  see  Wires. 
Stray  field  affecting  meter  accuracy, 

155 
Speed    control    by    rheostats,    see 

Rheostats. 

Switchboards:  A.C.,  2,  3,  162 
D.C.,  2,  162 
layout,  14 
Switches:  121-123 
costs,  195 

plug  and  instrument :  159-162 
ammeter,  160, 161 
costs,  200 

ground  detector,  161 
synchronizing,  160 
voltmeter,  159 

Problems  40,  42,  55, 56;  212-220 
representation  of,  5 
Synchronizing  A.C.  generators,  105 
Synchronous  booster  converters,  36- 

38 

Synchronous  converters:  34-38 
efficiency,  38 
Problems  16,  17;  207 
vs.  motor-generators,  36 
voltage  ratios,  20 
Synchronism'    indicators     or     syn- 

chronoscopes,  150 
Synchronizing:  lamps,  150 

plugs  and  receptacles,  160 
System,    choice    of,    see    Choice   of 
System. 


228 


INDEX 


!F-connection   of   transformers    and 

auto-transformers,  49 
Temperature    coefficient    of    resist- 
ance, 81,  83 

Three-wire  generators,   D.C.,  40 
Three-wire  lighting  system,  38 
Time-limit  relays,  see  Relays,  Pro- 
tective. 

Train  lighting,  58,  59 
Transf  ormes : 

constant  current  regulating:  109 
costs,  200 
Problem  36;  211 
current,    voltage,    see     Trans- 
formers, Instrument. 
instrument:  114^120 

advantages  of  using,  120 
affecting     meter      accuracy, 

155 

costs,  191 
Problems    38,    39,    44;    212, 

213 
current  (series  or  ammeter): 

116-120 
ratings,  120 
representation,  116 
on   series  lighting   circuit, 

164,  165 

Z-connected,  140 
voltage   (shunt,  potential  or 

voltmeter):  114-116 
connection,  115 
ratings,  116 

power  and  lighting :  43-50 
costs,  198 
efficiency,  45,  46 
frequency,  46 
grouping,  48-50 
kva.  capacity,  44 
overloading,  45 
Problems  2,  18,  19;  203,  207 
ratio,  43 

voltage  adjustment,  44 
voltage  regulation,  45,  46 
regulating,  109-113 
voltage  regulating,  see  Voltage 
Regulators. 


Transmission  and  distribution: 
A.C.:  85-91 

Problems  18,  28,  29,  36,  37, 

43,  49,  56;  207-220 
B.C.,  71-84 
Problems  25-27,    55;    209, 

210,  215 

Trucks,  battery,  58 
Tube   mill:    distance   to    center   of 

gravity,  182 
Problem,  56;  217 
Turbo-generators,  costs,  185 

U 

Utilization  efficiency  of  illumination » 
65 


Variable  loading  of  motors,  183 
^-connection,    power    transformers 
and    auto-transformers,  48, 
49 
Vector  diagrams  of  line  drop,  85,  86, 

89 

Vibrating  rectifiers,  see  Rectifiers. 
Voltage:  choice  of,  18 

control  by  rheostats,  see  Rheo- 
stats. 

drop,  see  Line  Drop. 
compensator,    see    Line    Drop 

Compensator. 
regulating  relay,  127 
regulators:  costs,  200 
generator,  126 
induction,  111 

Problems  37,  44;  211,  213 
transformers,  see  Transformers, 

Instrument. 
variation:  to  be  avoided,  12 

affecting  meter  accuracy,  155 
Voltmeter,  contact-making,  127 
Voltmeter  plugs  and  receptacles, 
see  Switches. 


W 


Ward   Leonard  system  for  motor- 
speed  adjustment,  40 


INDEX  229 

Watt-hour  meters:  149,  164  Wiring:  costs,  84 

costs,  189  diagrams,  see  Diagrams  of  Con- 

Watt  meters,  148  motions. 

Weatherproof       wires,      diameters,  rules  for  representing,  4 

weights,   and  carrying  ca-  Underwriters'  rules  (footnote),  9 

pacities,  80  Wood  working,  motors  for,  167,  173, 
Weights :  of  aluminum  wires,  80,  82  178 

of  copper  wires,  80  Y 

Wires :  areas,  diameters  and  weights,  y.connection .  auto-transformers,  31, 
80  49 

costs>  81>  84  transformers,  49 

current  carrying  capacities,  80, 

82  Z 

data  on   electrical  conductors,  Z-connection     of     current     trans- 
80-84  formers,  140 


1*49167' 


YC   19599 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


